I'm trying it out in server 2012 essentials beta now, and have found a couple of worrying things about storage pools.
1) Mirror / parity: you need to choose this when you first create the pool. You cant create a pool with one drive, then decide to add a second drive as a mirror. You must start with a two drive pool and declare one of them a mirror immediately.
Likewise, if you want to set 2 drives as mirrors, it looks like you need 3 drives to start with - you cant convert the pool from a no mirror pool to a one mirror pool, and then to a 2 mirror pool.
2) You can add drives to a pool easily, but you cant remove them. So, say you have 4 1TB drives in your server, and want to replace one of those drives with a 2TB drive. At present there is only one way to do it: manually remove one of the drives, triggering a failure, and then add the new drive and let it copy the
If you aren't using one of the mirror or parity options, this process kills the entire storage pool. Remove one drive, you lose the lot.
But even with mirror, this is kind of uncertain. There is no built in way to migrate the data off a drive before you remove it, the way you could with WHS v1.