During my latest exploration of the XBLA offerings I not only picked up Clash of Heroes but also stumbled upon a gem called Outland. Outland is a platformer that any metroidvania fan would feel right at home with. Throughout the course of the game the player slowly gains more abilities which in turn open up more sections of the map.
The real fun starts when polarity is introduced. Obviously drawing inspiration from Ikaruga, the hero can be in one of two states, Blue or Red. Or, in the mythos of the game, "good" or "evil." The important part is that projectile attacks don't hurt the player if they are the same color as the hero and only enemies of the opposite color can be hurt. This leads to a continuous on-the-fly adjustment of polarity between colors.
This is one of the few games lately where I've lost myself in the flow of the gameplay. Figuring out and executing complex patterns of polarity switching while running, jumping, and attacking gave me a huge sense of accomplishment.
As a downloadable title Outland is worth it's price. I would recommend it to any 2D platformer fans and especially fans of the metroidvania style.