What makes a published adventure great?
The short answer, and probably the truest one, is "great players". Including the DM, of course. IME, about 90% of the enjoyment of the game comes from the other people around the table, and everything else is very far behind that in importance. I would rather sit at a table with my friends and play "N2 The Forest Oracle" converted to FATAL than play through the best adventure ever in the best system ever... but have to play with a group of... well, you know.
Because a good group can rescue a bad adventure/game; a bad group cannot be rescued from themselves.
Of course, that's not terribly helpful, so...
But, in the most generic and open-ended sense of the question, what can WotC do to make you buy a published adventure...
Unfortunately, they're on a sticky wicket here. See, WotC now have a sufficiently bad reputation for published adventures that I won't be buying any from them for a long time. They will need to rebuild their reputation before I'll even give them a look. The problem is, of course, that if enough people stay away until they're good, they become a losing proposition, so there's no point in assigning the top designers, so the quality suffers, and so...
Their best bet is to more or less forget published adventures except in the DDI for the time being, and make sure DDI is good enough (in terms of tools and articles) that people feel they simply must buy-in. Then, relaunch eDungeon, but make sure they only ever publish the best possible adventures. "Good enough" is no longer good enough; they need to be stellar.
That should then get them sufficient exposure of their adventure offerings to rebuild a reputation, and then they can consider going for print again... if print is even a concern five years down the road.
what can WotC do to make... a published adventure {that} after running/playing it {you would say} that it was well-designed, definitely worth buying, and... recommend it to others?
(some edits by me for clarity.)
Meaningful choices.
Most of the 4e adventures (and even 'good' adventures such as "Sunless Citadel" or "Red Hand of Doom") lack many significant choices. You fight encounter A, then you fight encounter B, then you fight encounter C...
("Sunless Citadel", which pretends to have some choices actually has two: do you encounter the goblins or do you encounter the kobolds? and do you fight that first group, or ally with them against the second?)
In general, you can't even skip some of these encounters - in order to survive later encounters, you need the XP and treasure from earlier encounters in order to be of the requisite level. And, very frequently, the NPCs attack on sight, and fight to the death.
Ideally, the PCs should be able to choose from at least four ways to deal with the encounter: fight their way through, evade the encounter (eg stealth), deceive the enemy (eg use disguise), or corrupt the enemy (how about a nice bribe, eh?). Of course, not every encounter will allow for all four solutions, but most should, and every one really should allow at least two.
The third thing I think I would like to see, that would help move an adventure from 'good' to 'great' would be secrets to find - easter eggs for PCs to look for, puzzles to solve, and so on.
Now, puzzles in adventures have always been tricky, because there's the inevitable risk of getting stuck, and the game bogging down as a result. However, I think I saw the answer in some of those Lego video games...
One thing I noticed about those games is that there are at least three levels of puzzle. Firstly, there are the puzzles that are absolutely required to complete the level. These tend to be quite few in number, and generally pretty obvious - you know what you need to do, you have all the bits required easily to hand, so it's just a matter of doing it. Then, there's a second level for finding various stuff - you have to collect 10 canisters to get some Lego spaceship, or something. These aren't required, so it's no big deal if you don't bother, but for people who
like puzzles they're there and they're fun.
And then there are the easter eggs - little things that even the walkthroughs tend to miss, stuff that generally doesn't actually get you anything, but if you stumble across them (or if you're truly obsessive), there they are. (Like the disco floor in the first Lego Star Wars game.)
...
Of course, in amongst all that, what I think I've basically got to is that a great adventure should be strong in three pillars. At a straightforward level, it should have good
combats (I didn't actually mention that, because most adventures, in 3e and 4e, have actually been fine here). But where I think they've often been lacking, with some notable exceptions, has been in
interaction and, all too frequently,
exploration.
Hmm. Combat, interaction, exploration. I'm sure I've heard that somewhere recently...