I'm a pretty bad player, who picked up mons in Latimence, and though I quit mons before BW2, I've never had more fun then I did when playing with stall late gen 4, and post Exca Ban BW. For me The long methodical gameplay, and immense amount of thought that has to go into stall teams, is more enjoyable than anything else, and I sincerely hope it will be really viable coming into a brand new generation. Thinking about stall in a new gen is basically the only reason I logged on again, but as of now I'm a bit concerned with the way things are looking.
While I think some of the nerfs are very beneficially for stall, Mega Gengar is basically crushing my dreams. I simply don't see stall being able to dedicate a whole slot to dealing with him. You would need a scarf-user that can out-speed it and pursuit trap it after it's inevitably finished off at least one of your mons. I know he can be dealt with by means of say IB Bliss (who still falls to D-bond, taunt, and perish song) but he basically can't be countered at least to my knowledge with anything stall would use to help with other threats. To be able to counter M-geng you need to be able to trap him, and either out-speed or be able to take a massive hit. With his new found base 140 speed, the latter is out of the question without a scarf. If the poke you bring in on him lacks pursuit he can simply switch out and come back in later. Short of running Shed Shell on nearly every mon, I don't see how stall Bypasses him smoothly without resorting to scarfers with pursuit or maybe a dedicated chople or assault vest Tar.
Additionally the nerf to steel Types outweighs pretty much any buff stall received this gen IMO. Without the ability to resist dark and ghost, Alakazam, Gengar, Reuniclus, and other prominent specially based mons become much much scarier. A great remedy to these mons would be if Meloetta received a 1-turn heal this gen, but at best that's pointless speculation. Additionally pokes like Tyranitar and Scizor become even more troublesome with Crunch and Pursuit hitting for Neutral or SE damage against things that previously would have resisted it.
Lastly but not necessarily least, there are other troublesome megas to consider. Namely Lucario, Blaziken, and Charizard-X. While Charizard-X is a bit easier to deal with Regular or Mega Blaziken poses a pretty large threat with it's wide variety of options and access to moves like SD, fortunately hidden power nerfs probably help in dealing with Mixed sets. Lucario-X having adaptability and a base 150 atk in conjunction with base 145 spatk just seems absurdly hard to deal with. At best it's a 3 physical attacks set running either Crunch or Ice Punch, but even then guessing it's coverage move incorrectly could screw you for the game. Mixed or special sets are probably going to be much rarer, but still need to be considered. Mistaking one of the two for a fully physical set could be fatal for your team as well.
All in all I really want stall to rise to a position of power, but feel that it has a lot going against it right now. As of now I think some sort of pursuiting scarfer is basically required (or chople tar maybe) but one thing it has going for it is assault vest. I'm very interested in how that item will change things, and if it'll be picked up over the sustainability of lefties, or the usefulness of shed shells.