So, let me get this straight.
We have small, agile enemies--Crawlers--with quadruped frames (something unique to Halo so far) that keeps them even closer to the ground and out of our range. Taking elements from each of the light infantry we've faced so far, they can climb any and all walls, leap to heights bordering that of what Skirmishers could achieve, don't run away except in scripted instances, have very fast projectiles and they have no reason to stop firing, and they're usually fought in packs, not to mention upgraded variants which use even more powerful automatic weapons.
These, in turn, can be spawned by a Watcher in what appears to be groups of three at a time. Equally small in frame (if not smaller), they bob and weave similar to Drones, can reflect grenades, seem to have a fairly high level of health, can revive Knights, and project shields for them. This bears repeating: they can not only spawn new enemies into the fight, but also bring incredibly challenging enemies back to life as if it never happened.
Watchers are pooped out by Knights, who are by far taller and wider than Elites, but use every foot to their advantage, being battering rams, teleporting out of firefights, insta-strafing into your face to melee you, having jumps just as high as Crawlers, with a variety of weapons including a shotgun and the most powerful explosive, automatic, close-range and long-range weapons in the game, most of which are one-hit kills. They can use Promethean Vision to pick you out, and may very well be able to teleport through solid walls to get to your location. And these are common enemies whom you may encounter in groups.
Halo 4's campaign is going to be a bitch.