Now first off, let me say that I really like the new edition. I think/hope it represents a great new trend where GW actually tries properly to balance things and to fix loopholes. Overall, I easily I think it's great and well written (although I haven't read the fluff section yet, so who knows).
However, nothing is ever perfect, and trying to pretend so is foolish (go see some of the Fantasy 8th ed threads where people use the Reductio Ad Absurdio argument that if you dislike one part of the book you therefore automatically dislike the whole thing, etc etc).
So what would you do if you were given the change to edit the rulebook? Might be minor edits or going completely overboard and rewriting entire sections. Me, I might do this (in no particular order):
> Flying Monstrous Creatures only have to take their Grounded Tests if they take a Wound before saves
This is because I really dislike the idea that hits from blinky lights have not unreasonable odds of inflicting a S9 Ap2 hit.
> Fleshbane (X)
Just seems a bit of a missed opportunity to me to specify that the Fleshbane rule has a set value (2+ if unspecified).
> Hull Points to be increased by 1 across the board.
It just seems to me they've gone ever so slightly overboard on nerfing vehicles. I love that vehicles are no longer super tough, but it seems a little harsh.
> Land-to-Air / Air-to-Air missiles
I'd really have liked to see an option for missiles to be used against flying targets. Missiles are surely one of the best ways to kill them? Hunter-Killer missiles (or some new Skyfire variant thereof) would have been really cool.
> Actual definition between Fighters and Boys
Related to the above. I would have loved to see a bigger difference between Fighters (who should have some nice bonuses versus other flyers / skimmers etc) and Bombers (who should work better against ground targets). Not that they shouldn't work against the other sort, but just some bonuses would have been nice. Could be done in codexes of course.
> Power Axes at half I?
This one I'm not sure about. Making Power Axes the same penalty as Power Fists seems a little extreme. On the other hand, they're free.
> Zooming Flying Monsters to be able to assault flyers
This is kinda what Vector Strike does, however it would have been amazingly cool to see a hive tyrant landing on a storm raven and ripping its wings off.