Seriously, well, a Blood Angel and Necron alliance was already talked about in some fluff in the BA Codex. So, Space Marines and Tau working together may not be that big of a leap. We may also be talking about one or two sentences amid 80 pages of fluff in the 6th Edition Rulebook, so maybe it`s nothing to get all worked up about. Lastly, if Chaos is supposed to return to the forefront as the main antagonists, then this part about the Tau being brought to the forefront to combat them is not all that strange, I think.
About game mechanics. Some pre-measuring and some randomness is a good thing in wargaming...but too much of either can quickly make the game "un-tactical". We don`t know how much pre-measuring there will be; maybe it`ll be like just measuring from your HQ (like in WarMachine). We already have random charge ranges when charging into terrain, so maybe that isn`t so hard to fathom or accept.
Danguinius