Here's my concern about allies. I worry that we will start to see certain "ally packages" which are repeated over and over across competive armies from all codexes. Something like, Eldrad + a minimal unit of guardian jetbikes, to be used to completly shut down the opponent's psychic powers, and people realize this is a good package to have and suddenly you see 20 copies of Eldrad at a tournament. Or, a cheap guard HQ and troop squad and then a squadron of 3 vendettas (which might be able to come on the table 36" and shoot all 3 lascannons according to some rumors) being included in everyone's army that can afford it. That's the nightmare scenario.
BTW, the thing about Sisters, IMHO, is that they are misunderstood to be a bad codex. I guess it's a bad codex except for Celestine, Jacobus, Inquisitorial Henchmen, Retributors, Exorcists, Seraphim, Dominions, rhinos (better than anyone else's, 35 pts with a free 6+ invul save), and Repentia (OK that last one is debatable but I think they're pretty slick). Basically it's a codex with no choice but to take unexciting (though not terrible) troops, and then everything else in the army is quite good.