Well in fairness a lot of people's complaints about Halo were that there wasn't enough level variety.....the second half of the game has several sections where you basically retread areas already covered in the first half. This is an admittedly sloppy and rushed design decision meant to extend game length without producing any real "new" levels. Given that Bungie has had a full 3 years for the sequel the consensus is that you probably won't spend much time going back over old ground. Thus, that gives you something to look forward to.
I think that the strength of the gameplay in Halo makes the repetition forgivable, but Doom 3's gameplay is too flat and straightforward to excuse the same-ish levels. While you never revisit any old areas, everything has a very homogeneous aesthetic and archictecture. If you are the type of person that plays an FPS to see a good variety of locales, then Doom certainly isn't for you.....just as the later sections of Halo will probably rub you the wrong way.
The other serious gripe about the Halo multiplayer is something I don't really get. In LAN play it's a total blast. The weapon balance is particularly outstanding, especially when you consider that something like Doom is going to be dominated by nothing but the chaingun and rocket launcher. Even with Halo's stock pistol (& plasma pistol), you stand a good chance in a firefight even against the bigger guns if you are able to use the basic weapons right. If you only have the pistol you can still take down someone with a rapid-fire assault rifle or plasma rifle. In Doom, pistol vs. chaingun is going to be a blowout every single time.