Things that make a good adventure:
Most of us can remember the excitement of our first adventure, but if you've ever played in an infamous "hack and slash" campaign where all the adventures were "find the monster, kill it, get the treasure", then you know how boring it can be! "Hack and Slash" has a bad name, even among roleplayers, and for a very good reason - it is boring after the initial enthusiasm wears off.
Any type of setting, or adventure, will likewise grow dull, if that's all the campaign is made up of. Find reasons to mix it up... New and better swords (+1 Melee/+0.1 damage "Damascus Steel") in another city... if the PCs journey through the wilderness to get there, then find the weaponsmith to get them! "Healweed" that can double the healing rate, but it only grows in the lightless depths deep beneath the planet's surface. You can easily find ways to urge the PCs on, and use some of the following options, thereby. Even within a single city, there should be more than a single "flavor". The docks area, the hoy-paloy of the rich folk's "HighTown", the Thieve's Quarter, a section for foreigners (and maybe the non-humans), etc.
2300 is a game of Sci-Fi roleplaying, and it is the Technology that makes it fantastic. There should be some elements of such sort in almost every adventure. It need not be major, should not be central to every episode, but should be present. The new swords, above. A new item of equipment, a prototype to be tested by the Troubleshooters, etc. Maybe just a new form of Nanotechnology (especially one that the PCs have never seen, before).
Now in a game where Alien races more advanced than Humans exist, this new technology need not be Human, understandable, nor even functional. Even a few scattered Alien ruins may be of interest to the adventurers... and worth a bundle!
This may be as simple as "the Cavern of Singing Stone", which would react to living things and/or metal by changing the pitch of the "singing". This could simply be "neat", be used within the adventure as a way to warn a lower skilled parties of the approach of an alien creature, be a long-lost "Place of Power" for a vanished race... or even simply a neat new trinket for Merchant PCs to sell:
"Hey, buddy! Wanna buy a Singing Stone? See, they sing when you stroke'm! Neat, huh? Only $5,000!"
Not only does this help with #1, above, but it has the added advantage of allowing PCs with fewer skills to make use of the ones they have... This holds true for the skills needed to solve puzzles, as well (see below).
Again, this need not be the focus of the whole adventure, but merely background flavor. After each adventure is over, the players should feel that they have learned a little bit more about the world that they are adventuring on. The details may be minor (The national "color of state" for the Isle of Melniboné is yellow), but even minor details add flavor, and a lack of flavor produces blandness (which leads to boredom).
Some skills in the game (Combat Rifleman and Sensors, for example) have their own, immediate, built-in bonuses... Some others, like Leadership, Psychology, and Theoretical Science, do not. Referees should find uses for these "lesser" abilities within their campaigns, to allow those possessing them to not only make use of them, but wreap the rewards, as well. Perhaps in time, other PCs won't find them so "useless"!
In order for hero(in)es to emerge, there must be some form of conflict... In a more "citified" adventure, it may be Information Gathering, and not force of arms that wins the day. The Streetwise and Psychology skills may prove to be the keys to uncovering the information that someone else wants kept hidden... Computer may even be needed! Rivals vying for the heart of a Lady could be another form (with the poor-but-honest rustic farmboy hiring the PCs' help in protecting himself from the nefarious schemes of the slick, evil nobleman out to make him look like an Oaf - can the PCs expose the perfidy, without breaking the code of Chivalry and stooping to combat?)
Another example might be a group of 24th Century Quakers hiring the PCs to defend their settlement from an evil mining company that wants their land. The Elders refuse to allow them to use violence... The PCs have to figure out what the mining company is after, and foil them without fighting!
Quite a host of possibilities, here, and a wealth of possible "Scenario Hooks" for later use! If your PCs succeed in helping the Quakers, the poor, humble townsfolk should be grateful. They have nothing to offer (times have been bad), but they can (and should!) be glad to see the PCs who saved them, again, at some future date. If the PCs later fall into disrepute, why, the local villagers will simply not believe such scandalous lies about their saviours, and will happily offer the use of their loft/barn, and a bowl of hot gruel in the morning, to the down-on-their-luck adventurers who once helped them (after all... they understand hard times)! Even the lowliest housewife or scullery maid may be good for a bonus to the PCs' Local Knowledge, Linguistics (if they don't speak the local language), or other skill!
Another great thing about this (besides adding NPC personality) is that not all gains need be immediate... The rich old Lord may occasionally help out (when you're in his area), but the old widow won't, in any major way, until the GM decides she dies... Thus, "payoff" isn't until the Referee says it's time, and he can save this until just after a disastrous adventure, when the PCs are camping out, because they can't afford a motel!
Remember those old fairy tales? An ugly old woman on the road asks the proud travellers for a bite to eat, and six brothers turn her down. Along comes "the simpleton" brother, and gives her his lunch... In return, she gives him the location of a treasure guarded by magical beasts, and how to get said treasure, without being chewed up... One wonders why such simple acts of kindness aren't rewarded more often in roleplaying...
When a Doctor walks through town, does (s)he not see the poor, the halt, the maimed? What does their creed tell them to do? Ignore the problem? That true charity is to care for the fatherless and widows? That he that, having the things of this world, and seeing his (her?) brother in need, and helps them not, how dwelleth the love of God in them? How do the locals feel about such haughty strangers? When they find out he is a Doctor, might they decide that they have better uses for his supplies? (Ever see the show "Northern Exposure"?)
What if a kindly character helps some poor, old, maimed salt begging at the docks? This NPC could be a former Soldier-Sailor, and (having been down on his luck for so long) could then choose to follow and help/protect the only person in months (years?) who's showed him the least kindness... If equipped and treated well, the PC may have a new, able henchman (if the party is on the needier side), or just someone who's happy to see them, row them across the stream for free, and occasionally be used by the GM to provide a clue or some needed local information (if your PCs are more powerful). In either case, they should have a new friend, who can help them out in a later adventure... That, in itself, is a reward (and one that is all too frequently overlooked)!
In the example from the fairy tale, above, the old woman tells the Simpleton brother all about the creatures guarding the treasure, and how to get it without having to deal with them... If you ran such an adventure for your stingy PCs and no one fed the old woman, then you can have the guardians really chew them over, and eat half the party before they get into the first chest. Maybe there's a trap on the way out, too! Serves those rich, greedy PCs right, for not forking over a few biscuits to the starving old woman! Why, they probably spent more on arrows, bolts, bullets, energy cells, and grenades than the food would have cost!
This is similar to #3, above, but deals with your other (Non-Area-Related) skills. Allow someone with Information Gathering or Computer to gain clues through library research (who used to own that estate on Bone Hill, and what happened to them, anyway? Does the local burg have a Building and Zoning Commission? If so, do they keep FLOOR PLANS?)
Lost someone into another dimension, plane, whatever? Can someone with Theoretical Science figure out where they went? Are they dead, or just stuck elsewhere? Can they be gone after? Is it risky? Riskier? What are things like, "on the other side"? How do the PCs get back, once they've found their missing friend? So many kvestions!
As an example, players in one of my adventures had a very memorable encounter, which (until now) they never knew was simply a roll on the random encounter table... While they were setting up camp in the forest, I rolled up an encounter with a shapechanging creature, which I decided was in human form, to more easily spy on such a large party. The sentry spotted him with infrared, and sounded the alarm. The fleeing "man", was surrounded, and they then proceeded to ask what he was doing...
Wary of these obviously-potent adventurers, the shapeshifter stuck as close to the truth as he could, and told them he was a hunter who lived in these woods, and was just seeing what these strangers were up to...
Now all of this was made up, off-the-cuff, but the thing that made it memorable to the PCs was that I roleplayed the harried "man" throughout, adopting a pseudo-australian accent the whole time. One of the players (who didn't trust him at all, and was immediately suspicious - and rightly so!) said that he kept expecting to see a kangaroo come hopping through the woods... When a party member later disappeared (never to be seen again), the players really started to wonder who/what they'd run into... and never even knew it was just a random roll!
Even more care, obviously, should be put into personalizing the major NPCs used throughout the adventures: Motivations ("This alien wants to eat all of us? But WHY?"), dress, personality, etc. Even what weapons the NPCs use can be a link to adventure... Ever had a player badly wanting a +1 dagger, or sword? Think a +2 Katana would go over well with your PCs? Now who would wield such a thing (and possibly use it for/against the PCs in combat)? In one 2300 adventure, I had a small crew of very successful space pirates loosely based on Japanese tradition who used such things. "The Black Samurai" and his henchmen (including "The Red Ninja"), and their Katana, were the scourge of the space lanes.
In the murder episode (above), for instance, the players never seemed to notice that all of the murder suspects (the dead man's former trading partners) were females... Was that a clue? What can I say? The old man preferred dealing with the Ladies, and that part of his personality came out in his selection of trading partners. Yet another facet of his character was what finally got him killed!
Well, there you have it. Just a few basics, in my humble opinion... Not all of these need be present in every adventure, but each should make an appearance every few gaming sessions, at the least.