Halo 3 isn't really a bad game. I just don't think its very good. I loved Halo, the campaign was great, the multiplayer was fun (and only got more fun once I found Halo: custom edition) but Halo 3 really isn't any better than the first game. Sure the graphics are improved, but the campaign is not as fun, and the multiplayer seems the same. At least in the first one I can get more than 16 players playing, and some of the maps for it are still better (Death Island is still probably my favorite map. and there are a bunch others that I think are better than whats on Halo 3) I do like some of the features Halo 3 has, that most console games don't, including forge, but It just isn't that great, considering I could actually make maps for the first one, rather than buying new ones, when I got bored of the default maps.
Granted, thats just my opinion.