There are approximately 3.7 metric fucktons of published RPG modules out there. Which ones are good? Which ones are bad? Which ones make you want to light some candles and get out your nice pair of goat leggings? Fear not! For I have written a non-definitive, poorly sourced and quickly thought out guide to choosing the best of the best. Actually, I haven’t, but I’ll do that right now!
I like modules that have a good story. Really, you can throw everything else out if the story is cool enough (although that defeats the purpose, I’ll warrant ye). What makes a cool story? How should I know? I can’t tell what someone will find cool without examining their thoughts and feelings, and the last time I did that the judge was very unforgiving and the medical board wanted my head for practicing unlicensed medicine. But you know what you like. So go with that, I guess.
Mechanics? They should be solid relative to the system they’re written for. Solid mechanics are a sign of good editing and clear thought. Of course, to me, story trumps mechanics, but it sure is nice not to have to do a bunch of mechanics, yeah?
Good art is awesome. Great art is rare, and mindshatteringly good, like that foursome I had with (never mind). No! You shouldn’t buy a module just for the art, but good art, appropriate to the module, really helps tell the story. When you have something you can show the players, it helps set the scene in their minds. After all, a good mind doesn’t need art to imagine something; but then again, if your players had stronger imaginations they’d DM once in a while instead of letting you do all the hard work slaving away over a hot stove while they just come over and eat your food and don’t appreciate anything you do and never tell you how pretty you look and they didn’t even notice your new dress and that you got your hair done and…I digress. The point is, art helps build the scene (and expectations) up in the minds of both players and DM. It gives a solid basis to build off of.
Setting. This is a big one. Or not. A good setting, with good background, really helps immersion. If the story is highly dependent upon the setting and the background, then maybe you should really be into that setting. You can’t really take Lord of the Rings out of Middle Earth and expect to get any kind of good results. But you can take, say, Return to the Temple of Elemental Evil out of Greyhawk if you want. Change a couple of names, ehhh, no big deal. Paizo publishes Adventure Paths that could be adapted, but they really do work best in the Golarion setting. How much work you want to do is up to you. I like generic stuff, but then, I’m also fine with playing in one of the many published settings out there.
Tone. This is pretty simple. Don’t play horror modules if you’re afraid of the dark. Don’t play westerns if you hate cowboys. Figure out what the genre and tone of something is and be cool with it.
The internet is a wonderland of pornography and digital piracy. It is also, however, filled with the meaningless ramblings of untalented hack writers. But enough about me. There are plenty of sites that post these things called reviews which tell you what someone else thinks about something. You should look those up. Seriously. You might think you like the story, and are good on all the other categories, but if you’ve just glanced at something without purchasing and digesting it you really don’t know anything at all. Read a review or two. Can’t hurt, amirite?
Sometimes you just have a lull in your homebrewed campaign and you want to full it with something you don’t have to write. Fair enough, you lazy dog. This imposes certain restraints on what you can use (power levels, story tie-in, etc.). I’ve run into this problem many times myself. Sometimes you find just the right thing to give you a breather, and hey, maybe it really shoots some life into your game. Other times you won’t. Probably the worst thing you can do is try to shoehorn something completely wrong for your campaign into your game. I’ve done it, and let me tell you, I don’t care how nice a shoehorn you have, some things just do not fit into some places. Also, ducks are irritable creatures lacking in a sense of the adventurous, but let’s not go there. All I’m trying to say is that when searching for the right module, set yourself some boundaries and DO NOT CROSS THEM. It ends badly for you, your players, and any waterfowl involved. Know what you want and settle for nothing less.
I hope this has been helpful. Really, all of this has been said before, but hey, these are my thought on the issue.