Enlighten me.

[Arnwyn]

(aliens with not so good AI...)

Then you haven't been paying attention. The AI for the aliens in Halo are considered the best to ever grace a FPS so far. Even the much nicer PC FPSs haven't held a candle to Halo, with their somewhat lackluster enemies and stupid-as-hell bots (and a somewhat ironic statement, considering the two games you mentioned...).

So why do you like Halo (one or two)

- best enemy AI yet
- great controls for a FPS, considering there's no keyboard & mouse on the Xbox
- vehicles
- a decent story
- great situations and locations
- a reasonable level of "realism" (allies who actually help, battlefield tactics, etc.)
- 480p, widescreen, 5.1 DD
- co-op mode for the entire campaign

I'm not a huge fan of FPSs or the Xbox, and gushing really isn't my thing - but Halo really is that good.