Oh, there are many more features than that. But as I probably said (I'm not gonna read everything I wrote again!), it takes a while to identify Halo's little details that make it so good.
One is the randomized enemies. You won't realize it until you've played a level so many times you know every room by heart. THEN you start realizing that things aren't what you thought, you need to adapt your tactics, and everything goes to hell because you thought you knew exactly how to beat the game. A select few parts of the environments also have a bit of random to them. The last stone bridge on Assault on the Control Room has rocks on it that change places every time you reload the level.
There's also the rechargable shield, like I said, that ensures that you enter every fight with a fresh start. No more getting killed by the weakest enemies just because you had a hard time with the previous room. In Halo 1 this is slightly less so because of the health, but the shields still do a lot to increase the tempo in the game. Which is good.
Another feature that I haven't seen any other games of the type do is the headshot thing. Also expanded a lot upon in Halo 2. Certain weapons can kill an unshielded enemy with one headshot, no matter what enemy. In Halo 2 those weapons are the Battle Rifle, Carbine, Magnum and both the sniper weapons. Many games have extra damage to enemies for headshots, but Halo does it so very differently. You can take out the strongest Elite's shields with a plasma pistol overcharge, then just pick up the pistol and put ONE bullet in his head and he drops, despite being prevously completely unharmed, health-wise.
There's also quite a bit of open-endedness in some levels. More than you believe if you've only played the game one or two times. Especially on Assault on the Control Room and Silent Cartographer; easily my two favorite levels. There's a spot where you get a tank to drive around in and mow through everything, which is what one simply WILL do the first couple of times. But if you play it again, try snatching an enemy Ghost instead. Or even better, go on foot. Back then, it was unique for a game to do that. Usually in traditional first person shooters, if there is a vehicle around, you HAVE to take it.
Now I'll stop talking again. I need to eat something...