There's far more content in this beta than there are in pretty much every release. You won't be playing the same areas or maps over and over again. No, with how easy it is to make levels with the powerful editor, you'll be playing new content for who knows how long. Makes the Orange Box look like a ripoff in comparison.
The big story obviously is the level editor. Never before has such an easy to use editor been so very powerful. Usually with mods on the PC, you had to be pretty skilled to make them, and then people had to be able to find them. Here, everyone can make something, and the whole sharing system is integrated right into the game.
Quite simply, it's unique, and immediately gives the PS3 an identity. There's really no comparison to this game on any other system, which is what Sony really needs right now. Now, hopefully the game will catch on - it's very possible it won't even make the top 10 of the October sales charts. :(