Tobold has this post on his blog about making MMORPG combat better. It's a good summation, but missing one element, which could prove both tactical and interactive: coordination between players. Beginning in a simple manner, taking the D&D example of blunt weapons being good against undead as origin (that's new to me, never having played the game, but sure every game I seem to play has at least some influence from it), and letting some classes abilities interact with other abilities from other classes in a positive or negative fashion. Sure there is buffs in many MMORPGs that benefit more than the casting player but it's definitely worth considering how it could be taken further.
Here's an example: a highly armoured mob that cant be damaged by melee whose damage depend on applying bleeds/poisons, in order for them to be able to damage the mob the armour must be shattered/sundered by a 2-handed blunt weapon wielding tank. The tank is aided by a caster casting frost spells to make the armour more brittle and freezing the mob for higher chances of the armour being smashed, the bindings snapping etc. Once the armours gone melee have 2-3 secs to apply whatever bleeds/poisons they have, however the caster puts a 'hypothermia' debuff on the mob which prevents them being damaged with bleeds/poisons and this debuff is more likely to be applied when the mob is exposed for those 2-3 secs. It's the Zelda guide to boss design, the boss can't be ordinarily damaged only when a sweet spot is exposed and you can go hell for leather with your sword.
Another example is for stunning/knocking over a mob, (having just started LOTRO it'll be influencing this one, and the way burglars have the most ways to set off fellowship manoeuvres), say if you have 3 players able to charge they must time it to hit the mob one after the other and put the mob off balance and leaves them vulnerable to being tripped up and stumbling on some marbles some carelessly left lying around the dungeon (still cant get over that a Burglar's heal is called "Mischievous Glee" :). You could imagine other abilities that could throw a mob off-balance (I like the phrase 'the tipping point') to be tripped up, the smack about the head with a shield ability, the hurl abuse from the far side of the room ability and so on.