First of all, I don't own an Xbox, so the PC version of Halo was my first experience with the game. Second, even with a dinosaur of a PC (1.8ghz celeron and a geforce mx440), the game ran smoothly on 1024x768 resolution with mid-high graphics settings even before I patched it, and smoother afterward. So, while I disliked the game, it had nothing to do with missing features from the Xbox version or graphical slowdowns.
First off, the storyline. The list of games that haven't had a storyline of "lone man must fend off an entire army of aliens" is shorter than the list of games that have. While the story isn't awful for a shooter, it doesn't stand up to the likes of Deus Ex.
Second, the gameplay. While much of the game is fun, and the vehicle battles are great, the AI, even at the hardest level, is pretty weak. In the mid-game missions, I had a harder time keeping my backup soldiers from running stupidly to their deaths than I did killing the enemy - and, trust me, those soldiers are STUPID. At least in UT, you could give your AI team mates commands. In Halo, your "backup" acts almost entirely on their own.
As I mentioned, vehicle battles are usually pretty fun, but this isn't anything that hasn't already been done as well or better in Tribes and Planetside (it's one of the few things Planetside did right). However, the AI actually does a decent job here of working with you (not that it's that hard to program the AI to shoot anything that moves, as opposed to programming them to act intelligently). So, admittedly, Halo does have some strong points.
The last major problem I've encountered is the enemy troops. Even on the hardest difficulty level, enemy troops are no challenge. In some cases, it's not even that the AI isn't good enough; it's that the programmers made them do stupid things. A common scenario is walking into a room, killing one enemy, and having the rest of the enemies in the room run away flailing their arms in fear. I know I'm a badass and all, but I'm not so badass that an entire squad of enemy troops has to run from me just because I kill one of them. While I can see the point to making the AI intentionally stupid on an easier difficulty setting, it should have been taken out for the hardest setting.
Another, final note: Be careful when you patch the game. Apparently, the makers of Halo couldn't figure out how to keep the patcher from ruining saved games. I lost a save from 80% of the way through one of the longer levels because I took a break to play some multiplayer and it patched the game. It's sad that the developers couldn't settle a minor issue like that.