The free level editor and various mods, along with your pet casting spells and selling loot make this VERY re-playable.
Without TorchEd and the various mods I have downloaded, it might not be quite as fun, but I would still enjoy giving my pet spells and jewelry to upgrade it's abilities. Having your pet have it's own inventory and being able to send them to town to sell your loot instead of having to waste Town Portals is also a plus. And, (Blizzard, please take note) each item takes one slot in your inventory, no matter it's size.
And let us not forget the Shared Stash. Although there is no multi-player, you do have a special stash that all your characters can access. So, when your Alchemist finds that awesome axe that your Destroyer could use, you can get it to the other character.
I have already had a "Wait. I just spent 7 hours straight on this game?" moment. I just got off a three hour mini-slaughter before posting this.
I would definitely recommend playing this ($20 bucks? No problemo) while we are all waiting for Diablo III.