Man H:CE had some serious re-use of areas and back-tracking. I think the grind and cut/paste of The Library was worse than back-tracking, too much of the same for the whole lvl. Two Betrayals is my favorite lvl in the game, even though it's just AOTCR in reverse it's not the same repetitively and, the end battle is awesome. I think what really helped H:CE get more or less a free pass is it was the first console FPS to be as good/fun as it's PC brethren.
Given some distance from the original releases, I found all the games flawed in some way personally. I think how you view each comes down to how much these flaws impact you, your experience at the time of release and overall enjoyment of the sandbox now. The thing that surprised me the most when replaying is that the games vary much more than how I originally perceived them..
I actually enjoyed Halo 4 just as much, but I have to give kudos to the first one. It's just rare for something to hold up after so much time (especially since this was earlier in the 3D epoch). And I don't feel like the game was especially flawed at the time of it's release.
I enjoyed much of Halo 2, the new weapons were great, the vehicle physics were improved and the story was the only memorable one of the lot. But, I just found there was some sections which were just aggravating and poorly designed. Although I'm glad they had the dual wielding, I prefer the single weapon wielding and am happy that's how the series has moved forward in the present. But, it does feel like it throws the weapon balance off for me. The first one was more elegant in it's gameplay loop.
Halo 3 was by far the easiest of the lot for me (Heroic), but with no Elites and barrages of unarmoured Brutes and to a lesser extent the Flood, well, it just made it a
meh experience for me. The Unarmoured Brutes are a lame enemy IMO whereas the Armoured Brutes are hella cool.
Halo 2 was 2004, man & they were trying to do a lot on limited tech. The game originally was over 2x as big as when it shipped, they cut almost half the game out. I still think Halo 1's campaign was the most "fun," but you have to admit there's tons of back-tracking or repetitive re-use of areas in Halo 1.
Also, Halo CE can be more obscure at times, it's not always clear what you're supposed to do. I'm not sure if these things (backtracking as-well) are really a negative for me though. It makes it seem like it's me exploring the alien worlds and not just being funnelled through the game linearly (but this is more perception than reality). This is how games used to be and I kinda wish there was a bit more of that in modern stuff.