Ok so every once and a while (although lately it seems to happen more often…Ratchet and Clank 3) I put on my QA hat when I start playing a game. This usually happens when I start seeing problems, Halo 2 while a good game is not perfect and to me feels rushed.
Some of us have been going back and forth about what is good and in my case what I do not like about Halo 2; here are my impressions thus far.

AI of Enemies: (tested on all difficulty levels)

Almost from the Start of Halo 2 I have noticed that the AI just is not as good as in Halo.
Example(s): Elite soldiers in the first and second levels would every once hide behind some type of cover, while this is cool, I was able to strafe around the cover look them in the eye, polish the barrel of my assault rifle then shoot them. The whole time they were just standing there.
In the City level many of the Enemies have a tendency to walk into me more often than take a shot, and in the “Sewer” the patterns of ghost pilots is just out right formulaic.
While I do not see this problem often I expect more from harder difficulty levels other than shots taking off more shields.

Graphics:

While Halo 2 has made improvements on textures used on vehicles, and Chars the over all feeling on each level is that of “muddy”. Everything outside of you reticule seems flat and lifeless. While many levels have moving parts to give them life, I do not feel the sense of immersion I did in halo.
On multi player maps this is most apparent that was probably due to the amount of stuff going on when you factor in poss. 16 players on 1 map with effects. I just get a stale feeling in Halo 2.
The games frame rate is consistent, but I have noticed some slowdown.

Cut Scenes:
This is just a quick point, the cut scenes seem rushed. I am not saying they are bad or horrid looking, but they just look strange from time to time.

Honestly those are my only major faults with Halo 2. I still think it is a very good game, and I am having fun. I am a little disappointed because it feels like MSFT rushed this game out the door. Halo to me was just about perfect, but I keep telling myself that if you factor in the original Mac Dev time and the time spent porting to the Xbox, then fine tuning some more Bungie had close to 5yrs to work on Halo.
there is a good post from /. that goes into some of the issues in Halo 2. I wish Halo 2 had been given just a little more time; I will still probably play through Halo 2 5 times as I did in the past with Halo, Bungie did a good job but there are just a bunch of little things that keep it from great.