Some mobs get additional abilities on Tactician. For example the turtles in Act 1 explode in flames at death which they don't on Classic.
Mostly it is higher stats however. Which make the game harder and fights more interesting, despite what many moaners say, because mobs with more armor and life live longer to apply their higher damage, so you have to try harder.
Of course better mob tactics and so, the wet dream of all pro gamers, would be much better, but there is only very little "I" in all AI we have through all games available.
