I think the nerf to horses would be alright if it were done well. Is there any way for somebody to code horses to have their real-life gait speeds? At the moment we only have the option for maximum velocity, but It'd be nice to see people walk, trot, canter or gallop their horses in towns, along roads or on the battlefield. It doesn't have to be crazy, just gallop and walk speeds are fine - say doing /walk will make your horse slow down as if you were to use the command on foot?
As for custom nexus arrows doing half the damage of a normal arrow, you may as well just remove custom arrows all together then, 'cause there'd be no incentive to make them in the first place.
I think the 1.9 styled combat should stay in for all warhammers, but that's just my opinion.