These are just my suggestions, feel free to comment/add to/ridicule them

Charging
It occurred to me whilst we were playing that there is a bit of a loophole when it comes to charging Tau that I dont think they picked up in playtesting. I figured at the time that I might have just been misreading something, but ive since gone back to the rulebook to confirm it. A strict interpretation of the rules as written makes it quite easy for a Tau player to make it entirely impossible for a unit to be charged. Ive not looked at other armies for similar situations, so it might not be unique to the Tau.
Basically, the problem comes from the rules about who you can charge. The rules state that a charge must end with at least one model from the attacking unit within 1 inch of a model from the target unit, and no model from the attacking unit can be within 1 inch of a model from an enemy unit that isnt the target unit. If that's impossible, the charge fails and the attacking unit doesn't move. That seems reasonable, except for Tau drones. Drones are deployed as if they are part of a unit, but it explicitly states that once deployment is done the drone is no longer part of that unit; all drones are treated as individuals after that point. With the number of drones Tau can field, and the fact that they start with units, it's not hard to mingle them up with your front line of, say, fire warriors so that there is no point on that line where there is a fire warrior that isn't within an inch of a drone, and vice versa. That means that there is no way for any models from a charging unit to end up within an inch of their target unit without also being within one inch of a non-target unit. This essentially means that with some careful model placement you can make a Tau army entirely immune to charges. That doesnt seem like an intended rule.
I think there's two ways to fix this:
1) Treat drones (and similar units if this crops up in other armies) as attached to the nearest friendly unit for the purposes of working out charges.
2) Alter the rule slightly so that instead of it being that you can't end a charge within 1 inch of a non-target enemy, have it so if that would occur you have to target the closer unit for the charge.
Cover
We covered (haha) this a bit at the weekend, but the short version is that the 8th edition cover rules are a bit bollocks. As written: a unit gets a +1 to its saving throw if it is *entirely* within a terrain piece. Which is fucking stupid for two reasons:
1) if 9 men in a 10 man unit are in a building but the tenth man isn't, the entire unit gets no cover bonus.
2) It only allows for being in things. Being behind a bloody great wall that covers half of your model gives you nothing.
We kind of reverted back to the kind of cover rules in SW:A, but I think that could do with a bit of tweaking to make it work better (ie faster without being bollocks) in 40k. I suggest the following:
You work out cover as though the unit were a single model (for expediency, and to reflect the fact that in reality there would be a bit of moving about going on). Check LoS from the attacking unit (again, as if it were one model, even if it isn't) to the target unit. If you can entirely see the target, no modifier. Up to half the target unit obscured (whether that's because you can see the top half of every model, or because you can only see half the models) gives a -1 to hit. More than half obscured gives a -2. Entirely covered (or if only a gun/antenna/hat is showing) means you can't target them at all. Obviously that last bit depends on the model: if you could only see the gun on my Ghostkeel I would probably still allow the shot, as its a bloody big gun. Personally, I think that gives a good compromise between being more believable than the official cover rules without bogging the game down too much by having to work out which individual models in each unit can see what percentage of each model in the unit they are targeting.
Thoughts?