I completely disagree with the idea of locking out difficulties. I personally play the hardest difficulty first time on any game that I play, as I feel this presents the most comprehensive challenge. If the hardest mode is locked, and I have to play the entire game to unlock it, I am losing out. This is because I have already experienced the game. I know, to an extent, what to expect and therefore lose an element of the challenge. Whilst locked difficulties make less of a difference to those that like to play on normal mode (at least first), it is detrimental to people like myself, that want the biggest challenge from the game that is possible.
I think Call of Duty: Modern Warfare had an interesting idea though. All difficulties were available from the beginning, but with a little training level that suggests a difficulty based on how you play. Nothing is forced on the player, they still have their choice, but there is a little hint at what to expect. I quite liked that idea. I just ended up practising the level until it suggested Veteran mode for me, moving on, content that I was ready for the challenge.
To sum it up; if I've paid money for a game, I want to have the choice to play it my own way.