Exclusive Dataslates
There's only really the Skyhammer and SM Command Tanks that are feasible given the BW points limit etc. I think the Skyhammer could be abused but I'm prepared to allow it this time and hope that people resist making it too cheesy.
Flyers/FMC
Both of these units have been around for a good while now. The Tyranid codex is actually one of the "older" books actually. They're horrible to deal with but not unstoppable and I think to limit their use would hamper Tyranid players too much. I really hope people will avoid spamming them too much but I'll judge this on an individual list basis.
The main thing is to bear in mind that you might face one of these lists and plan your army accordingly. That means either taking plenty of anti-air or simply deciding to ignore them (as far as possible) and focus on their ground support units. Remember a Swooping FMC or Zooming Flyer can't claim an objective.
Special Detachments (e.g. Decurion, Warhost, Gladius)
Additional special rules aside, I think the main problem here is only certain factions have access to these detachments and it allows them to bypass the BW restriction of two detachments/formations. Until all (or the majority at least) factions have access to them I don't think they should be permitted.
To clarify though, this only applies to those detachments which are made up of formations. This includes things like the Champions of Fenris detachment from the supplement of the same name. Basically if it's just a single detachment/formation made up of units (in whatever combination) it's fine. If it's a detachment made up of subformations it isn't. I'm sure there'll be stuff I haven't thought of but if you're unsure contact me.
Conclusion
Generally speaking my stance this time around is going to be minimal restrictions and instead a (perhaps misguided) trust that you guys will enter into the spirit of the event and not bring total filth. I'm sure GW will throw plenty of spanners into the works before BWX comes around but I'll try and stay on top of things as best I can.
If there any things you think I've missed that are particularly awful in your opinion please feel free to comment here or contact me by email.