I don't mind having some locked characters, but locking content up has gone overboard in recent years.
Raving Rabbids is the worst offender: you get the game for partying, but you can't play 90% of it without playing the stupid single player - which has its moments, but it's still a bad design for a game that's mostly about partying.
Less than 50% of the game should be locked up front. In fact I'd say it should be more in the range of 20%-30% unlockables.