So I just ran software update on my macbook pro, anxious to check out the new app store.  Looks like what we are very familiar with in iTunes and in the app stores on iOS devices - with one exception.  For me, it wouldn't let me download jack squat - it just kept popping up "Error 100" over and over whether it was a free app or not.  Fortunately, fixing this is simple - here's what I did:

• I ensured I was signed out of the Mac App store by using the "Sign Out" menu option on the "Store" menu.
• I quit out of the App Store completely.
• I loaded iTunes and went to the iTunes store.  I picked a free app for my iPhone and downloaded it.  If you're looking for free apps - there are several from my company, GeekUtils (click here).  You can download a paid app too, doesn't matter what you download - the key is to force iTunes to pop up the new "Terms and Conditions" so you can click agree.
• Once you've agreed to iTunes new terms and conditions - exit iTunes and restart your computer
• When your computer restarts - load the Mac App store.  You should be good to go.  If not - try clearing cookies from your safari cache, which appear to be shared to this app.  Then, reboot again and reload the app store.