It's somewhat obvious that enemies take more of a beating in Guacamelee!'s hard mode, and I seem to recall some bosses' attacks being changed to undodgeable. What other differences are there, if any?
Bosses had new tricks.
X'Tabay has 2 smaller platforms instead of one large one in her dimension swap. Her final phase also has three images instead of two.
Flame Face summons much harder minions to deal with while you fight him.
Javier seems to be noticeably faster in hard mode.
Calaca don't seem any different, other than dealing more damage.
AFAIK nothing else.