A series first. According to a now pulled GamesRadar article, Resident Evil 5 will feature a cooperative player available throughout the campaign which lets players drop in and out of co-op play. The new cover system mechanics will let players "lock to a wall and fire around the corner." When not in co-op model, the other character will be AI controlled. What's more, the game will feature a dodging movement controlled by the right analog stick, and the game will take place in more open areas than previous RE titles.
Resident Evil 5 [GamesRadar via Shacknews]