UAC prompts for a start are bypassed. You have the authority in that user account to delete anything, including system files and folders. It is extremely dangerous for this reason as you may, if you work fast, click the wrong thing and bingo - no operating system.
If your machine became infected while you are signed in as Admin the malware has free rein to do just about anything.
Here's an article mentioning why you shouldn't use it, it's talking about reasons to sign in to the Admin account but I am sure there are lots more.
I have the Admin account enabled and on my Welcome screen and is available for me to sign into by switching users should I need it, but I would never use it by default.
Vista is a much safer system than those that went before and offers warnings when something you are doing may have drastic effects. By using the Admin account you are niftily avoiding all that.
Also see:
10 things you should know about User Account Control in Vista and....
This tutorial from this very forum which contains the same warning I'm giving you:
http://www.vistax64.com/tutorials/67567-administrator-account.html
Create a user name for yourself in the CP, with Admin privileges if you wish. Not quite the same as the actual Admin account but safer. It's an easy matter to transfer personal stuff from one user to another.