As explained above in terms of AI Halo has it easy, elites are able to drop back behind cover and recharge their shields so they either have to be dealt with at one time or you keep fighting them over and over. I have no problem with this, perhaps this does encourage the player to be more proactive and creates depth in combat, i.e. to break the stalemate. Nevertheless, soldiers in games like HL2 are not afforded the luxury of infinite recharging shields and therefore the AI must be considered on these merits. Unlike elites which have the same health and shields as MC, the average soldier in HL2 is dead in a single shotgun blast and Gordon could probably take five point blank before going down. This gives the HL2 player a big advantage because you can simply rush the enemy and blow them away with minimal penalty like practically every other FPS ever created. So Halo is unique in this regard, the best enemies are your equal and since they're often lying in ambush this can set up a good challenge. As I said though, not every FPS can be designed under this 'infinite recharging shields' principle and if one is going to compare the AI of Halo's seemingly 'amazing' enemies to those of other games then one should take this into consideration.
Not to say that Halo's AI isn't without its flaws, for example, watch in awe as an elite in the open pointlessly dodges from side to side as you simply circle round back and pump round after round into him until he's dead. This sort of dodging shit may work when your ducking behind cover but in the open it's pretty stupid. Then there's also the small covenant grunts who will often turn around run away in the middle of a fire fight, pretty stupid as they're simply mowed down like grass afterward.
The fact is, despite what bullshit people may have talked about HL2's AI, the enemies will seek cover, they will attack in groups and attack from various angles to flush you out and will often times completely outflank you. They'll throw grenades at you, be proactive in their environment such as running over and using an unmanned minigun and enemy gunships will attempt (with much success) to shoot down missiles you fire at them. Your AI team mates will also give you (and each other) health and ammo and move out of the way when you need them to. Also important is that the AI does improve the further you move through the game, and having to duck for cover and use objects in the environment as cover becomes common place. To tell you the truth though, when you look at the quality of other aspects of the game, from the graphics and animation, the very well paced and varied gameplay, great sound and effects and not to mention the amazing integration of the physics engine, the AI is not up to the same sort of standard by comparison. This does not make it bad (it is certainly above average) but I don't think the AI is as good as it could be. Still, to outright call it bad and discredit the game because of it as some have done is totally ridiculous and shows premature, incorrect or (most likely) biased judgement.