The only negative aspects with the K90, at least to me, is:
A: It's not fully mechanical. I can make do with the F1-12 and page up and down keys being non-mechanical, but the Delete, Esc and macro-keys should have been. If Corsair ever makes a revision where they replace those keys with mechanical switches I'll pick it up in an instant.
B: The macro pane, no matter how you put it, looks like an after-thought. The aluminum bezel ends, and you get this rough, sturdy plastic instead. The keys are lower too (good for feeling difference between the G1-18 keys and WASD I suppose).
C: Back-lightning is only blue. This is a premium product, and as such I feel that the back-lightning should either be of optional colour (i.e. 3-4 different versions with red, white, green and blue) or simply with a RGB switcher.
D: I guess headphone-jack and an additional USB port would've been nice but... I personally don't need it.