Final Level: Assault on Capital Prime
Why It Sucked: You've managed to do what no one else could—infiltrated Capital Prime, harnessed the Authority satellite and reactivated the Arks, ensuring you'll have allies in your burgeoning rebellion. Yet Rage's final mission is linear, cramped and repetitive, and the abrupt ending hints at awesome things before ensuring you'll be back for the sequel.
How We Would End It: Cut out all the pointless fetch quests in the middle of the game and add half their length's worth of decent missions onto the ending. Gathering up the other Ark survivors and launching a full-on assault on the Authority would have been satisfying as hell (but then what would the sequel be about?).