Imaro
Legend
I think a paragraph or two of flavor text for a monster is the right amount. You can fit a lot of ideas into that length, as the mind flayer entry demonstrates. 4e gets it pretty much right, imo, as does 1e. Those ecology articles in Dragon magazine? Six pages on the ecology of the trapper? Very not right, imo.
I don't really think it's about length... and I don't think it necessarily has to be the same length for every monster. I will agree though that alot can fit into two paragraphs... if the right information is presented, I have no problem with that length, and honestly I don't think most people are asking for 6 pages on each and every monster... moreso I think people want better as opposed to longer background info... though in some cases it may be necessary to expand it in order to get the better story stuff.
Out of curiosity, which is your favorite 4e MM?