>But the interface change is not that significant compared to all the other new features
It might not be the most significant feature from your PoV, but I think it'll do wonders for the adoption rate of GIMP. Its one of the major obstacles that people experience in those crucial first 60min of GIMP use.
I liked the multi-window interface, but the choice of making the palettes real windows and not floating panels always struck me as a mistake. See how Pixelmator handles that for example: it's much more natural.
I don't know Pixelmator but I think there's an option for that. In Preferences->window management you can change it from regular window to utility window, then it's just a question of whether your WM cares.
But thankfully there's a choice between the two interface modes. The GIMP crew should have come up with that earlier.
But the interface change is not that significant compared to all the other new features (esp. >8bpp color).