A co-op mode that's supposed to be a lot of fun, too. I'm not 100% but I think it might be only adhoc, though. Hopefully I'm wrong.
Beyond the Co-Op, it also has a fun VS mode. It's Local Only on 360, but did they make it online on PS3/V? Haven't heard.
At any rate, it's akin to a sidescrolling Bomberman MP match. It's pretty fun, and a worthy part of an already fantastic package.
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No one should doubt Spelunky being worth "all teh moneyz". It's one of the most mechanically-refined, fun 2D games in recent memory, has great music, and multiplayer Co-Op and VS feel great together.
The random has enough rules to where it feels like "an army of solid stage design" rather than "jumbled junk that's tossed out to artificially increase playtime".
And the HD art actually manages to meet or surpass the charm of it's old pixel-origins, unlike many other games, which lose much in the process. There's a nice organic nature that keeps the tiles from being super-repetitive, and the characters personality within their few little animations try a lot harder than your average "one step above flash" 2D now-a-days.
Add crossbuy atop all of that, and I'd assume the inclusion of the DLC Characters and VS Stages that were released on the XBLA version a while back, and you have quite a bargin...