Terrible GUI lag/glitchiness after reboot in Mountain Lion 10.8.2
I successfully resolved the glitch/lag problem on my Retina MacBook Pro by re-installing Mountain Lion without use of the Migration Assistant. I have outlined my steps below:

Backed up all my data to an external hard drive
Completely powered off my MacBook
Turned on my MacBook while holding down the &quot;Option&quot; key
Entered the Recovery partition
Used the Disk Utility to reformat my SSD
Re-downloaded and re-installed Mountain Lion
When asked if I wanted to use Migration Assistant, I opted out
Manually reinstalled software, copied over files, etc.

I really think Migration Assitant had something to do with the lag. Either that, or some file or software I had caused the lag, and Migration Assitant simply brought the problem over to my new MacBook when it migrated all my files and settings. In any case, my MacBook Pro is now buttery smooth again, with none of the lagginess and graphics artifacts I had before. I'm happy!
Just for the record, here's all my MacBook Pro with Retina Display system information:

Mid 2012 model, 2.6 GHz Intel Core i7, 16 GB 1600 MHz DDR3 RAM
OS X version 10.8.2

I hope this helps anyone else with the same problem!