Call me crazy, but I loved the Arbiter.
One of the things that bothered me most about HALO 1 was that we had no idea who the Covenant really were. "See they're these guys. Yeah, these guys, from another planet. They all look different from each other, for some reason. They've got crazy weapons and purple ships. And, um, they want to take over Earth. We really don't know why. Oh, and they have a religion of some sort."
I'm told they explain more in the books, but, well, I never read them.
Thanks to the role of the Arbiter in HALO 2, we now have a much better understanding of the Covenant, the way their society works, their religious practices, and the significance of Halo and Earth. Actually being able to play as an Elite does so much more for the game as a whole. Now the Covenant are not just these generic aliens, now they seem like actual characters, with real depth and dimension. We have a reason to hate them or even empathize with them (I found myself seriously rooting for the Elites over the Brutes, which is very strange considering the number of times I've yelled "FUCKING ELITE!" at my TV during my HALO career).
And besides, optic camoflauge at the push of a button? Cool. Hunters and sword-wielding Elites as your allies? Very cool.
So that's just my two cents. I'm surprised that the Covenant levels are taking so much heat, they went over really well with our crowd.