By Firestone I love expansions. LOVE THEM. Sometimes they can "fix" some of the problems that emerged in a game when it first came out. Sometimes they can just breathe some new life into a game that has grown a little stale. Sometimes they allow more players to play the game (although this is rarely a good thing, IMHO).
Age of Steam is a good example of a game ripe for expansions. Awesome base game, and the expansions are just maps. Maps of new areas—real and imagined—with new and interesting rules and mechanics. Does it always work? No! (cough*Golden Spike*cough). But when it does (Montréal Métro, for instance), it makes me love the base game even more... (Power Grid also benefits from more maps.)
Card games are a natural fit for expansions. Thunderstone, Nightfall, Netrunner, and Lord of the Rings all benefit from just more awesome stuff to add!
Pandemic's On The Brink added some significant gameplay changes—including someone playing as the baddie! Those are the most risky expansions, because they have the potential to be awesome or terrible.
But sometimes expansions are just...awful. The first one that springs to mind is the expansion for The Princes of Florence. Now understand: For years and years and years Princes Of Florence was my very favorite game. It was only recently eclipsed by The Resistance, due to the sheer amount of fun I've had with it. So I was excited to play with the expansion that came in the Treasure Chest (one box that had 10 expansions for six games). It. Was. Horrible. Our one and only game using the expansion took 4 hours. 4 HOURS!! Toward the end of that game, the expansion made me hate Princes Of Florence. Any expansion that makes me hate my favorite game is bad, bad, bad.
Another bad one was the Necromancer Island expansion for Small World. This was a freebie giveaway promotion, so I think they felt they could experiment a little. It's lame. It forces the players to cooperate against the Necromancer player—which doesn't really work well in the framework of the game. But beyond that, those who do work to fight the Necromancer are in a worse position that those who don't. Blech. It's going for ~$30 on the secondary market, thanks to completists who didn't get it when it was free and want a copy now. I'll happily part with my copy for that price...
So what are some of your favorite expansions? And what are some that fell flat for you?