A mobs health can no longer be above 1024. Running this command -

/summon Skeleton ~ ~1 ~ {Attributes:[

{Name:generic.maxHealth,Base:25000}

],Health:25000f}

will result in a skeleton with a 25000 "maxHealth" attribute, but only 1024 "Health". Trying to force an /entitydata @e[type=Skeleton]

{Health:25000f}

also doesn't work.

This makes creating boss fights in maps a lot more difficult (especially bigger ones) as a boss with only 1024 health goes down in around 30-40 seconds flat with only a good enchanted diamond sword. In 1.8 there was no such cap to worry about as a map maker.