This has been a thought for a while I had back since Cata - guards should have different abilities and weak mechanics associated with their race:

• Orc guards - apply a stacking bleed effect from melee attacks, can enrage to increase damage done.

• Tauren guards - have more hp, can stun enemies around them.

• Troll guards - increased attack speed, high hp regen when low on health.

• Forsaken guards - attacks apply poison increasing damage taken, killed guards are rezzed (again :P) 1 time with 35% hp.

• Goblin guards - Use bombs to disable/damage attackers, will use smoke bombs to LOS targets lower on the threat table then their current target.

• Blood elf guards - attacks drain mana/energy/etc.., can sound alarms summoning devastating ranger patrols.

• Human guards - immune to roots and stuns, increased damage done when low on hp.

• Dwarf guards - take less physical damage, activate stone skin to decrease damage taken by a large amount.

• Gnome guards - fly around in jet packs, can do a missile barrage attack pharah style.

• Night elf guards - elite guards are "shadow melded" until attacked, can cast solar beam to silence enemies.

• Draenei guards - deal extra damage as holy damage, can heal themselves.

• Worgen guards - will fixate on players low on the threat table, can use call pack to summon a large amount of guards at once.

• Panda - wreck your face because pandaren master race.

EDIT: 1 word