Comment 1 for bug 1291851

I've just tested a:
 NVIDIA Corporation GT218GL [Quadro FX 380 LP] (rev a2)
on trusty (fully updated) and Mir works with nouveau better than ever :)

A few questions then:
  1. Are any packages on the system non-standard (built by you)?
  2. Do you accidentally still have any traces of a proprietary NVIDIA driver installed?
  3. Does X/Compiz work well on this system? (i.e. Unity blurs work).

Maybe try reinstalling all the official Ubuntu mesa packages? And when possible, reinstall the whole system fresh from a trusty daily image? Because all signs so far indicate it's most likely a problem with your installation.

When I have time I will try another Nvidia system.