Old 3rd ed style screening--like putting the concealed guardians in front of everybody--works again. But a lot of the time it isn't necessary because pretty much everybody gets a cover save anyway.
Vehicle changes are big. It's pretty much impossible to hide a landraider anywhere on the board, but in order to hurt it with like a guardian-platform brightlance I have to (1) roll a 4+ to hit (2) roll a 4+ to damage it (3) hope my opponent rolls less than 4 for it's cover save and then (4) roll a 5-6 or 6 to destroy it. That's 4% or less.
Odds are a bit better for a wraithlord brightlance, and better still for a twinlinked brightlance (especially if it's on a serpent and can move to get a clear shot). But clearly melta weapons from up-close or assaulting with something str 9 (witchblades) or 10 (wraithlord) has a much better chance of killing armor 13+ than any long-range antitank weapon--especially if manned by guard or Eldar with BS3. It's not so much that tanks got a boost, but that long range antitank weapons got nerfed.
Running is also huge. I actually saw a space wolf army on foot run across the board with minimal casualties and assault in turn 2-3. Assaulting across a broad front minimizes the effect of not being able to consolidate, and running troops move so fast that they can bypass speedbump units. I see some viability to a wraithlord with just a wraithsword and a couple of flamers, especially running up and bashing vehicles.
Also the countercharge ability is strong. With space wolves, Bloodclaws in particular, because they can run up into your face and even though they can't assault, they still get all their attacks when you assault them (and they get the extra hand weapon attack). All kinds of chaos marines, except thousand sons, would be good at this too.
Plasma missiles fired by guardians actually hit stuff. Because you don't have to roll to hit. Cool.
Assaults are decisive and much faster to resolve. It used to be that high-leadership units like necrons or seer councils knew they'd never be testing on worse than a 6--but now those units can be trying for snake eyes because of casualties. Being in the same combat with a lot of weedy mooks--like guardians--makes dedicated assault units more vulnerable to breaking. Because if a lot of your weak troops go down and you lose the combat, then even the elite assault troops are testing on a 3 or 4 due to casualties in the weak unit.
Overall I like 5th ed. 4th was starting to get stale with all the optimized power armies, so it's nice to have something different.





