Most old 3D games have aged horribly. 3D graphics just don't age like 2D ones do. A good 2D design will always be a good 2D design, even if it gets a bit pixelly when you're used to higher resolutions, but a good 3D design 5 years back is almost always a hilariously bad 3D design today, and the further back you go the worse it gets.
Gameplay-wise, how well a game ages is, in my opinion, a very strong indicator of how good the game really was in the first place. If a game was good five years back, it'll still be good today (unless your taste in games has changed completely, which isn't really the point). However if a game was great five years back but is boring/awkward/badly designed today, you've gotta wonder why you liked it in the first place.
For example, I've been a big Jagged Alliance 2 fan for pretty much as long as the game exists, and I cn still fire it up now and then and have as much fun with it as I did when it was new. It simply is a very good game. On the other hand, I still have this idea in the back of my head that Command & Conquer: Renegade is absolutely the best game ever to play during a LAN party, but when I play it these days it just doesn't interest me that much. This makes me re-evaluate my opinion of the game: Did I like it because it really was the perfect LAN-game, or just because the LANs I played it at where that so fun that the actual quality of the game didn't matter much?