As a D300 user now, I would find a D400 worth the extra money over a D7100, if it had a faster continuous shooting rate and a larger buffer than the D7100, along with the other features that come with a D300 body. I don't need 24mp, and if they upped the frame rate and buffer size by making it a 16mp camera, I'd even prefer that to a D7100. When you consider how much you pay for the system -- camera, lenses, and all -- a few hundred more dollars for the camera isn't that big a deal.
That said, the D7100 looks like a very good camera to me. I'm committed to sticking with DX (because I have a very useful set of DX lenses that I don't want to give up), and if it happened that as I look to upgrade I was "stuck" with a camera like the D7100, I wouldn't be too unhappy about that. Many of the things I like about my D300 (autofocus, 100% finder) have made their way into the D7100. The D7100 looks like a very good camera for the money. I'll even leave out the last part of that and say the D7100 looks like a very good camera, period.