I think they meant Xbox 360 only when they said "platform exclusive". You know, exclusive to the Xbox platform instead of the third party games that seem to be running the show right now. Since the release of the first Xbox, the Halo series was number one in sales, online play, and overall popularity. It was the big dog in the world of Xbox. Now, even though Halo is still the biggest exclusive series on Xbox, it's getting beaten by a couple of third party series when it come to sales and being the most played on Xbox Live. I think M$ wants Halo to be number one on Xbox and they feel that Master Chief will be the reason that this will happen. No flames being thrown at Nintendo or Sony at all.
Unfortunately, Master Chief is a boring and generic character. He has the personality of a robot and he appears to be wearing green moto-cross gear. Halo 4 is going to have to do a lot to make him a likeable character because the game has always been the star and Master Chief was just a vessel for the player. The games haven't really gotten worse. Just too much of a good thing and that can be boring. I hope 343 is up to the challenge.
