Just a heads up, this isn't the official site of 343i. http://forums.halo.xbox.com/
But AFAIK, and as far as anyone is concerned, there are no button combos, nor should there be. It breaks balance. A skill gap may need to be present, but giving advantages to players who know how to press certain buttons really fast isn't balanced and enlarges the skill gap too much.
And uh, competitive isn't binded to a set of rules. All game developers can tell you, competitive multiplayer is any multiplayer where there are opposing sides. Casual multiplayer can encompass co-op, or competitive multiplayer. Competitive isn't a defining group between good and bad. Anyone who plays any playlist in the Competitive or Community playlist is competitive. That's just the cold hard definition.
What YOU mean by Competitive and Casual is like Ranked and Social playlists in Halo 3, yes? The people who don't quit and try their best? I can tell you, the ranking system should make you want to win, and make you feel like a loser for quitting. With what we know of it, yes. There will be that sense of what you call "competitiveness" in the game again.
Rest easy, the BR only has recoil, no Bloom. The DMR does have bloom, but it is so minuscule that you do not have to worry about it. The little "X" shaped markers that you see in the gameplay videos are just hit-markers that tell you whether your shot hit a target or not. Bloom has always been in Halo, but it isn't visible in any Halo before Reach. Remember how you could fire your first AR bullet on target on Halo 3? Then they'd start a small scatter? That's bloom. but it isn't made visible by the reticles. In Halo 4, any bloom is only visible because it shows you how precise your shot will be, not to make your shots scatter more than they should.