Yes, I think that putting in the nominal bitrate is a great idea, as it gives people some inkling of what they're going to get. Especially when they move it to 128 and then find out that it sounds a hell of a lot better than that mp3 that they had from the same bitrate.
Some notes though, just as UI quirky things go:
The quality reading shouldn't be an edit box unless we can edit it. I was a little surprised when I hit a key and it jumped to 10.0. Changing that to a label would be more intuitive.
Is it possible to not have so many tickmarks in the slider? This is nitpicking, I know, but it would be nice to only see 10 ticks on the slider.
Managed bitrates should probably be stuffed under another dialog box, or perhaps an "Advanced >>" button that expands the dialog itself. A message should be in this new area (preferably not a dialog box) stating that quality settings sound better and that you shouldn't be using this stuff unless you know what you're doing. Maybe even a web link if someone's feeling particularly saucy.
[Edit]
Realized this right after posting: If we make the managed bitrate settings LOOK intimidating, users who don't know what they're doing will probably be much less likely to fiddle with them. Sprinkle in some technical terms 10 cent words with the warning that your files will sound worse unless you're some kind of guru, and it might work. It does for me, at any rate.
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Finally, is there any particular reason that the default quality setting is 7.5? Maybe it's not, and I changed it and wasn't paying attention. But if that's the case, perhaps we should consider moving it back down to 3.0 to match oggenc.
Yeah, this is probably over-engineering when the most used encoders will probably end up in Audiograbber, CDEX, or whatnot. I think it's a Good Thing to be the shining example of what to do for the programmers of these programs, who may use OggDrop as inspiration (especially if everyone goes "look how they do it, it should be that cool!"

).