I'd like to give this Game Circle thing a shot, so I'm just going to jump right in, if that's okay.
I first played Halo a few months after it had come out. I didn't buy it, but was just playing it with a friend at his home a bit. While I was fairly impressed with the implementation of a pretty good co-op mode, I didn't think much of the game at the time (I wasn't really thinking about it critically, at that point) and let it become somewhat forgotten.
Fairly recently, I picked up the first two Halo games again, and played through them both (both solo and with a friend). Taking a more critical approach, this time, I noted a few things that made the first game a pretty darn good first-person shooter when it came out. The real over-arching thing to keep in mind is that Halo doesn't really do any one thing particularly amazingly, but it really is a darn-good, full-package game (something Keiths Dad mentioned above).
As you've all probably noticed by now, Halo is a pretty action-centric game. There can be quite a few enemies assaulting the player at any given time, and they can be doing all sorts of different things. A good comparison here is with another well-known console shooter, Goldeneye. Comparing the combat sequences in Halo with those in Goldeneye really demonstrates just how many more combat angles there are in this Xbox behemoth. The sheer number of things going on in a battle can far exceed the scope of many earlier console shooters. Then, you add vehicles into the mix. As if the fairly complex shooter terrain in the game isn't enough, you've got the implementation of fairly well-done vehicle combat. Primarily, this allows for a lot more mobility in a combat situation, both for the player and his opponents. What this all does is give Halo quite a bit of combat depth, particularly in the larger, outdoor environments. Indoor environments in the game, in general, feel a lot more constrained and mundane to me, and I think a lot of it is just contrast with what the outdoor offers.
I also have to say I quite like the exposition. It's not intrusive, as it can be in something like a JRPG (see: Tales of Symphonia), and it tends to happen in running, in-game dialogue, or in fairly brief cutscenes that don't happen overly often. The result is (with a nod to Dyselon who noted this earlier) that the game tends to flow from mission to mission extremely well. I find that the flow and atmosphere tend to be helped along by a darn good supporting soundtrack, which is always nice.
Granted, there are problems in the game. AI problems show up fairly frequently in the more complex battle situations (which really isn't a problem unique to this game). The difficulty settings can make the game damn hard, but the balance sometimes seems out of whack - some battles become head smashingly frustrating when ramped up, while others really aren't much more difficult at all. You can create some framerate issues at times (especially if you're trying a speed run or something, where you're not killing all the enemies before moving on), but they aren't particularly frequent.
Also, the thing about the game not really doing one area brilliantly can make it seem bland or unoriginal, especially if you're a fan of certain types of first person shooters. If you're a PC gamer, you're probably not going to be overly impressed by anything in the game. However, I'd challenge you to put forth another game that wraps as many of those things into a good, cohesive package as Halo does. Even with the fairly notable selection of excellent PC shooters, there aren't many (even now, years after Halo: Combat Evolved's release). On a console, it'd be news to me if there were any that came close.
To be honest, I would have to say I usually would prefer a game that excels in one area to one that does a lot of stuff pretty well (I like to think Plato would approve of this state of mind). That said, Halo: Combat Evolved must have surpassed whatever threshold I have set forth, because I found it to be pretty enjoyable.
