If a buddy account is anything like a trial account, there are a few restrictions:
-Level restricted to 10( After you hit 10, you still get xp but you wont lvl up anymore. Once you hit approx lvl 12, it stops completely)
- You cannot trade items with trial accounts
- They only last for 7 days?(or 30? it usually says with regards to the trial accounts)
Those are the 3 major things i can remember about those accounts.
I think buddy accounts are like trial accounts but you can lend them your CD's and you already get the CD key for the trial in the game package.
Makes it easier for your friend to install and begin a trial, but i dont think they are anything more special than regularly downloading the trial.