Aaron Margosis has excellent set of posts on why and ho to run as LUA (non-admin). If you wander what's in it for you or have particular problem while running as non-admin, you owe it to yourself to read his blog.

Here's a short list of Aaron's posts on the topic:

First the “Why” posts:

Why you shouldn't run as admin...
http://blogs.msdn.com/aaron_margosis/archive/2004/06/17/157962.aspx

"Zero-day" attacks and using limited privilege
Expect to see more malware predating the patches - and how you can protect yourself. (Or, "Why you shouldn't run as admin, Part 2")
http://blogs.msdn.com/aaron_margosis/archive/2004/06/25/166039.aspx

And then the “How-To” posts:

The easiest way to run as non-admin
This is the really important one for your non-techie friends and relatives ...
http://blogs.msdn.com/aaron_margosis/archive/2004/06/17/158806.aspx

"RunAs" basic (and intermediate) topics
A whole lot of detail about how to use "RunAs" to run programs under a different account.
http://blogs.msdn.com/aaron_margosis/archive/2004/06/23/163229.aspx

RunAs with Explorer
How to get Windows Explorer to work with RunAs (and why you might want to).
http://blogs.msdn.com/aaron_margosis/archive/2004/07/07/175488.aspx

MakeMeAdmin -- temporary admin for your Limited User account
How to quickly and temporarily give your non-admin account administrator privileges, without having to log out.
http://blogs.msdn.com/aaron_margosis/archive/2004/07/24/193721.aspx

PrivBar -- An IE/Explorer toolbar to show current privilege level
A toolbar for Explorer and Internet Explorer that shows you broadly at what privilege level that particular instance is running
http://blogs.msdn.com/aaron_margosis/archive/2004/07/24/195350.aspx