I personaly would prefer to never see the Birdman in any Doom game, please NO.
The Arachnotron, however, I can't wait for (Doom4).
Also, ToJKa, I don't like that Arachno model, looks reall funny to me, I preffer this one:
http://media.moddb.com/images/mods/1/7/6603/phobosaction_1_1600x1200.jpg, looks MUCH scarier and more badass.
Lol, and actually I can't wait to fight the Pain Elemental again, I've always thought Doom was pretty easy, and then when the Pain Elementals came I got myself a challenge more

The deal with Birdman:
I think in fact the only reason some people like it is because of the eyes, yes. The beak looks ridiculous, and the whole shape of the head with "feathers" going back look really funny, not scary.
However, those huge yellow glowing eyes, 2, looking at the player, create that huge fear of a monster from Hell, as Marnetmar explained.
Basically, change the Doom3 monsters just a tiny bit and you can get the result you want. Just look at this Hellknight with eyes and horns (which the creators named "Baron of Hell"), and you get that "nostalgia" back of the classic Demons, which alot of people want back:
http://media.moddb.com/images/mods/1/11/10042/baronofhell.jpg
(Of course, it'd be cool if they had changed more of the body too, but oh well.)
Yep. Other than the face, Birdman looks really funny. Instead of asking for Birdman, you should just hope that Doom3 brings back the classic style demons with "more demonic faces."