I wouldn't say it's empty as far as features & content goes. I played quite a bit of this after finding it on Greenlight about a week ago, and it's more enjoyable to me than Nidhogg or Towerfall. They're constantly looking for more map ideas, and have been working away at Streets Of Rage-esque four player-coop levels, filled with AI enemies & objects to be used in your attack. They're also doing work on more moves, such as headbutts, and healthbars for those of us who prefer beating our adversaries into a pulp.
As simple and fun as the game is/looks, I would treat it like more than your average time-killer. It has a lot of potential to become the next, indie-fied Super Smash Brothers. Regarding the lack of features and content, it's still in a very, very early stage of development. Not to invalidate your opinions, but I feel like saying you could play it while waiting for SG, only to throw it away afterwards, is selling this game short completely. It may not have incredible, historically-accurate settings and a fully fleshed mêlée combat system the way Sui Generis does, but it's very clearly on its way to become a lot more than a free, downloadable game à la generic Ludum Dare entry.
