I don't think the rules are very 40k-ish at all, personally. The fluff certainly is, though.

It's all on a tile system, distance is measured by tiles, or is ignored in favor of just plain line-of-sight. You get a certain number of points every round for each model, which can be spent on moving, attacking, opening/closing doors, and (for the Marines) overwatch, which makes you auto-shoot whenever a genestealer moves within your LoS, and defend, which helps you in close combat should you get attacked. There's no leadership, no armor saves to speak of, flamers set the room on fire (and it stays that way for the rest of the round), you can move each model separately, and the mission objectives typically involve more than just "king of the hill" or "king of your stuff."

As has been mentioned, the models are fantastic, and are precolored to match which side you're on (they're not painted), the tiles look really neat and seem pretty durable, and I learned all of the rules in about twenty minutes in one sitting.