Attack runs. Every time you want to use that expensive cool jet you dragged out of reserve on turn 4, it has a 1/6 chance to tell you to fuck off. It doesn't do anything but irritate people who field cool jets.
Demo, particularly first turn demo. It definitely shouldn't be removed, but I don't like the way it encourages drive-on and slaughters infantry in ways flame and CQB can only envy (except for A4 dudes and Sirens who don't seem to mind so much).
Flame weapons only being able to target a single base. Overkill should transfer to a different base in the same unit.
Both the Desolator and the Overseer to an extent. Both nice concepts, but it falls apart when you remember that one is shit because it needs babysitting yet actively kills its babysitters and the other is basically mandatory because it makes a powerful combo that cannot be done any other way. Both fancy rules need some reworking.
Ferrum. Leave it alone for now, UCM would be garbage without it. But as soon as DZC 2.0 comes along that thing needs to be dragged back to the drawing board and reworked. Infinite number of anti tank anti skimmer anti air scout drones with MF15" is pretty fucked, especially in a faction all about specialised units working together to get the job done.
Destroyers. They don't die to machine guns, or flamethrowers, or falling masonry, or CQB, or basically anything. Fuckers have the shielding of a Shaltari commander.
Panther. Unlimited range AA. Fuck.
The way focus weapons work. Right now they're effectively E13 weapons that can occasionally split apart to fight lighter targets, and I don't think that was the intention.
It's still a great game, but personally I can't wait for DZC 2.0. You can tell that Dave and the team have learnt a lot with how smooth DFC is (aside from a couple of dud units and slightly bullshit special rules), I'd like to give them a chance to start fresh with DZC. Smooth out the bloat, and really look at each faction with a purpose.