Mass Effect is the first installment of a sci-fi themed RPG trilogy from Bioware. That alone is enough to get my interest. The story centers around mankind's early steps into inter-galactic space and encounters with alien civilizations.
I expect the game to have the same branching story paths, fraught with moral choices, as Knights of the Old Republic or jade Empire did although those did not ultimately have any effect on the story itself. The player will travel through the universe on his own starship and lead a squad of characters through military style missions.
Combat will be handled in real time on some sort of tactical level with NPC's making 'intelligent choices' about what actions they should take.
The game should also have some replay value. Instead of being limited to exploring just a few planets Mass Effect promises to let the player explore much more broadly possibly through an element of randomly generated content for planets outside of the main story path.
If the game were not solely for the 360 my interest level would be much higher. But prices may be coming down before the holidays and there is also Assassin's Creed to consider. Possibly those two games will be enough to finally convince me to buy the console.