Some of the pitfalls of windows 7 M3 build you need to be aware of
The two major ones are
Losing Sleep (If you load all your daily-used applications and use it like a production OS)
Whenever my windows 7 laptop goes into sleep or hibernation, when it tries to wake up, you get BSOD. On a bare minimum Windows 7, you won't get this.
Unlocking the hidden features of Windows 7
www.withinwindows.com shares that there are hidden features in the M3 build that is not available to people who are not under Microsoft's payroll. Workaround are given to unlock those features, and they work. However, don't do it on a machine that is shared by various people. The unlock steps I believe changes the permission of explorer.exe. On my wife's laptop, after I unlock the feature using my account (domain admin), and I log in using her account (normal user) you will get a blank gray desktop. Basically, normal user account do not have rights to run explorer.exe. Changing the permission does restore the desktop, but not to full functionality.
Minor stuff
Other minor issues do happen, especially when you are installing drivers and applications. Most of them can be solved by running the installation in Vista RTM compatibility mode.
EndPoint Protection won't install properly too. You need to manually install life update first, before installing EndPoint protection. Even then, proactive thread protection is not enabled initially. Just antivirus. After running it for a few days, Endpoint protection will return to normal operations.