Character slots if you're an altoholic or want more than two characters per server. Of course the limit is on creating characters per server, not on the total number of characters per server. I've got a character limit of six per server, but because I had six alts on a server that was being shut down and six alts on the server that all characters from the first server were being sent to I now have a dozen on the remaining server.
If you can find a valid product code or boxed copy of the base game, or the Storm Legion Collection which includes the base game, you can unlock all the default character and bag slot limits right off the bat. Of course that doesn't involve the in-game store, or even Trion's site as they're no longer offering either, so if the idea is to grind platinum to buy WREX in-game and then unlock account limits the idea's a bust.
"Before I send them to hell, I shall let them see heaven." - Claptonvaughn, http://www.gamefaqs.com/boards/605432-path-of-exile/68145584#11