Having played the alpha, I can say that in The City, we were confined to a single area on top of a tower, divided into three spaces, and there seemed to be at least one area that was blocked off, but the playspaces in the alpha felt EXTREMELY LIMITED in an INTENTIONAL WAY (The area in Old Russia one could explore and play was HUGE, but it easily felt like there will be much more to it yet still, and that's just one "play space"). So I wouldn't be hideously surprised if there will be more of The City to explore, but then it is intended to just be a sort of quasi-hub-world of sorts. But you CAN walk, jog, and sprint in The City, and even crouch walk or do the slide moves, and jump (but not double jump) anywhere. Walking/ jogging speeds feel fine. Maybe Halo 2 or 3 in terms of feel. Sprinting doesn't feel as fast as it was in Reach, but does get you where you need to go quickly and isn't limited in duration or use. There seemed to be only one "sub-class" per class in the alpha, and undoubtedly there will be more from the videos, but depending on the power, you can change out different perks for certain things on a whim (through a short series of menu options that are intentionally lengthy enough to not be pleasant to deal with in a live firefight, discouraging people from messing with stuff when you need them defending points or the such and so on), with no limitations, but there is ONE PERK that locks your choices (seemingly) permanently IF YOU CHOOSE TO DO IT but in exchange gives a pretty hefty stat boost to the character.