Speed is one of the most critical factors with regards to momentum. Look at all the things that primarily exist to change a pokemon's speed - Tailwind, Choice Scarfs, Trick Room, Paralasis, etc. Dragon Dance and shell break are so threatening because it makes the sweeper so fast. Outspeeding an opponents is a great way to threaten it with something and so force it out, and allows you to respond easier. I think that we are going to need something that can somehow outspeed threats.
A good example of this is scarf Latios or Doryuzuu. They're so much more threatening because they are faster than just about anything out there and they hit hard. They wouldn't be nearly as threatening if they weren't so fast. Latios can come in (on revenge, or a resisted attack) and force out just about anything because it is going to outspeed it and do a good chunk of damage. It's an immediate threat that easily responds to threats by either forcing them out or killing them. It also forces your opponent to play predictably (go to steel or blissey or lose a pokemon).
Hell is an exclusive club. Don't let the riffraff in.
Watch the Leaders of Yesterday teach the Children of Today how to run the Country of Tomorrow. Into the ground.