The best way to look at it is like this;
How much fun would it be to combat a singlular enemy in an empty room with your weapon of choice. In the case of Halo, I can honestly say that fighting most of the enemies in a 1 on 1 environment could actually still prove rewarding and fun. The enemies and weapons of HL2 would not provide anywhere near the same level of enjoyment due to the way everything functions.
Now, in the context of the game, we all know that you generally do not face enemies in this type of situation. The point, however, is that if the combat is good enough to the point where the above scenario becomes enjoyable, it would absolutely shine when placed in an environment (as long as the rest of the AI is up to snuff).
If you face one of the more powerful combine troopers in that room, what would you do? If you were facing a Covenant Hunter in that room, what would you do?
My point is that, in the case of the combine soldier, I'd probably just aim for the head and should a couple times. With the hunter, I'd move within close range, lead him on a bit, wait for him to charge, dodge his attack, and fire a pistol bullet right into the small patch of exposed guts. The method used to fight that combine solder would apply to most enemies in HL2 (you just shoot towards the head until they die) while the attacks used to kill the hunter simply would not work in the case of other enemies.
In Halo, weapons combos are possible and encouraged, CLOSE COMBAT is EXTREMELY deadly and fun (not even present in HL2), enemies require VERY SPECIFIC strategies to kill efficiently, etc etc. HL2 doesn't offer this kind of combat. It's entirely different and nowhere near as addictive to me.