Halo 2 is in my opinion, the best game ever. You should deffinently play, it if you haven't already.
The one thing that I really was surprised about the game was that it was improved over the first. I thoight the first was as great as it could be already, but Bungie proved me wrong. The dual weilding and vehicle boarding made the game alot more interesting and intense. It's great to just steal someones vehicle because you don't have and it's even better when it's a banshee in mid-air. And then there are all the new weapons they added, some may not be as useful but they have their place. Then there are the weapon combinations that can be very usful.
Another great part of the game was the Arbiter. He brings in the Covanent side into the game, although it may not make you like them any more it does help you undestand them. And that there are some who are going against the high councel of the covanent. And he brings a new element of stratagy to the game with his cloak. It is so incredibly fun to use his cloak and run up behind an enemy and hit him in the back or throw a granade into a group of bad guys. Or even getting a good position to shoot from can be obtained without being noticed with the cloak.
The levels in Halo 2 are much more open and diverse than the first. With all the different locations you never get board of what you see. There are even a few hidden places with stashes of weapons or power ups or maybe even a vey nice snipering spot.
The AI are much improved and can be very handy. I had a marine with me once in High Charity who I had given a Carbine to and he lasted half the level on Legendary and now they have learned to drive.
Some people say the storyline is bad but it is one of my favorite parts of the game since I am a big sci-fi fan. All the twist and turns kept me interested all the time. It also expanded my view of the Halo universe considerably.
It is absolutly my favorite game ever!!!