Simple, one thing. Taking what PC gamers had for years, and after many others had failed managed to push the gametype sucessfully onto consoles, and made (arguably) as good if not better than PC shooters.
Halo2? For good and Ill, Matchmaking system/DLC value vs return precedent.
Halo 3? Uh.... uhh.. Bringing the mapmaking mod system that PC shooters had for years, and brought it to viability on a console? Thats all i got.
ODST? Uh...pass
Reach? Jetpacks?
Not a big fan of Halo. I respect its well made, and things like its collision detection and ragdoll physics are surprisingly refined. I can every once in a while sit down for a few hours some random afternoon and do some multiplayer, but I really do not get, and never have got what exactly is so phenomenal about the series that sends some people into such an uproar. Its well made, its polished, it has its place in gaming history, but really that would be a paragraph, not the 3 book epic some people would have you believe (dont think any game ever made can claim such notoriety )
Having played the beta a little bit.. it looks like pretty much the same thing you would expect out of halo.. heavy emphasis on the multiplayer aspect with a solid multiplayer matchmaking system, a slightly refined graphics that given the simplicity of style in game you will notice for about 5 minutes and then forget, and... jetpacks.
It do like seeing the concept of styles of toon. I would like to see them progress it much further, such as several different types of armor sets that do differing things such as regeneration, heavy defense at the price of speed, vice versa, Pulse wave discharge armor, ect and keep with adding the more RPG-esque elements to allow people more options in customization and personalization.
I think reach will end up being one of the better additions to the franchise. And I do know that it will do well sales wise, but it wont be from a heavy influx of new fans, its basically going to be the same fans who will make it sucessful, who bought the prior 4.5 titles in the series, which is good, but it is a clear sign of the series showing signs of stagnation.