Producer from wikipedia:
In the music industry, a record producer (or music producer) has many roles, among them controlling the recording sessions, coaching and guiding the performers, and supervising the recording, mixing and mastering processes.
so, obviously Nigel is responsible for the final mix/sound of the album. he mixes and re-masteres it. many songs sound different live and on the album, and Godrich responsible for it, i mean for studio versions (Idioteque, The Gloaming, 15 Step supposedly etc are better live undoubtfully. so probably these are his most recognizable mistakes, although 15 Step isn't finished yet). imo, the final version of the album depends a lot from the actual producer...
so that's why he's "the 6th member" and he's done a really great work with Radiohead, without doubts, but he also made hell a lot of mistakes, most of them on the last 2 albums (HTTT, The Eraser). i still like him, and he'll stay great for me because of The Bends, OKC, Kid A/Amnesiac but he needs to re-new, i just have a feeling he has no motivation at all or i don't know...
i really hope the LP7 will prove that i'm wrong.
note: seeing Radiohead with another producer could be more exciting because i think they already reached the 10.0 with Nigel and not only once, a change would be good, maybe
but then again, we still haven't heard the new LP, so it's too early and unfair to blame GODrich already now