Monstrous creature and cover:
There are no special rules for monstrous creatures and cover any more, which means they gain cover like infantry. So aside from the 25% rule, monstrous creatures also get area terrain cover AND from intervening models! (That means termagaunts now give trygons 5+ cover, crazy)
Transport vehicles and units charging:
Units being transported can never charge on the turn they disembark (apart from assault and open topped vehicles). That also includes vehicles that have remained stationary OR vehicles that have been exploded/wrecked in the previous turn!
Assaulting a unit in ruins:
If a unit is attempting to charge another unit in a ruin on a higher level, but there is no space to fit the model, it is perfectly acceptable to place the model on a level above or below and still have them counted as being engaged in base contact. Supporting attacks can be make within 2 inches on the same level, and 6 inches from another level.
Regrouping falling back units:
If a unit is falling back and under 25% not 50%, it may still rally on double 1's. If there is an independent character in the unit, the unit may always rally on his leadership.
There's some I can think of right now; please add any of your own if you find them.