I know I've made a big argument about this in the past, but one of the main reasons why I find the Library (and Halo CE in general) so incredible is the level design. The (seemingly) miles of endless corridor and the constant wave of Flood really create a sense of confinement and brooding that really suck you into this alien structure.
To me alien architecture, especially in the context of a Halo, should be big, long, endless, and repeating. These structures were not designed by HGTV, they were designed with very specific purposes by a very specific alien race that ALWAYS built on a monumental scale. I really feel that Halo CE nailed this whereas the latter two games did not, at least to the same degree. Tsavo Highway, The Ark, The Covenant, and the on-foot portions of Halo should have been longer, in my opinion.
ICO is another great example. Sure they could have made certain rooms smaller, certain bridges shorter, but that would have taken so much away from the overall feel of the game. Halo CE and ICO achieve a grand sense of scale based on their level designs, not based on their skyboxes and distant geometry that you cannot reach.
Halo 2 and 3 feel more like corridor shooters to me than their great-grandad.