I love when mob casters displace themselves and their melee companions, when fire elementals cast resist cold as their first action, when enemy clerics lead off with a deathward then do the unbelievable of healing their allies.
If epic mobs were casting deathward, my pale master would be slotting break enchantment and mordenkainen's. If the epic clerics recast when it's dispelled, I'd be focusing CC on them before my dispel.
Simple AI improvements would necessitate skilled game play.
And in cases where you want a challenging encounter rather than trash, add one or two red-names in the mix and make them tough.