My bugs on Windows 7 / Server 2008 R2 so far:
Twin Monitor setup
Vista introduced the great feature working on dual monitors, where you could simply drag maximised window from one monitor towards another, and...voila...window moved to another monitor, and it was also maximized there.
In Win7 / 2008R2 it does move to another monitor, but window gets restored to smaller size.
Multiple RDP sessions windows
When working with multiple Remote Desktop sessions opened towards some other computers, RDP sessions simply vanish after some time. Only 1 (random) RDP window stays opened (most probably the one you were last focused on).
Let me just confirm, that I tested it on multiple computers, and all Windows 7 behaviours were much alike - RDP windows were dissapearing randomly.
Windows Live Messenger
In idle state or even not logged-in, WLM ocupies 2 active icons in taskbar. One icon showing thumbnail of my WLM profile, and 2nd icon, showing actual WLM login window.
Also, when you do not work with WLM, some instant messages will not trigger neither any sound, neither will taskbar icon blink when somebody writes you a message. You need to click on WLM window and only then all messages will blink. seems like a bug in taskbar focusing.
IE8 back-buttons (and others)
I am not sure if this is Windows7 / 2008 R2 related, but I noticed this behaviour only on newly installed machines. IE8 sometimes freezes buttons for BACK and REFRESH, and also TOOLS menu is mostly grayed-out.
Seems like some security feature, which is buggy.
Taskbar buttons
For advanced users, it is handy to have also some non-standard shortcuts pinned to taskbar. For example, I want to have shortcut to NETWORK CONNECTIONS list pinned to taskbar.
To call NETWORK CONNECTIONS, you need to create shortcut to:
%SystemRoot%\explorer.exe shell:::{21EC2020-3AEA-1069-A2DD-08002B30309D}\::{7007ACC7-3202-11D1-AAD2-00805FC1270E}
Ok, and beside this one, I also have shortcut to WINDOWS EXPLORER.
Now, when I run NETWORK CONNECTIONS shortcut, it becomes active in taskbar, but it also activates Windows Explorer shortcut, showing them both as active.
Networking
If you configure 2 NIC adapters, let's say, one for INTERNAL and one for EXTERNAL traffic, Windows 7 / Server 2008R2 will (wrong!) put them in the same network neghbourhood (Private, Public...) if NICs are from the same vendor (for example, if you have computer with 2 NICs already built-in).
I had to buy additional NIC adapter to make EXTERNAL and INTERNAL network treat separatelly.