This page is a discussion of the Racial Traits of the races that can play as the Mage class, and how much they can offer a mage.
Alliance
Gnome - gnome traits
Gnomes are especially well-suited to caster classes, with a 5% bonus to Intellect, and a bonus to arcane resistance. Also useful is the Escape Artist ability, which is invaluable when the low-hitpoint mage needs to get away from creeps or players with movement-hampering abilities.
The bonus to engineering is nice if you want to explore the strange devices produced by engineering, however explosives (a large subset of engineering recipies) are of dubious use to a mage with a full spell arsenal. There are plenty of other interesting things, however, and there is no shortage of gnomish mage engineers.
Human - human traits
The human's abilities of The Human Spirit, and Diplomacy are useful for any character, and the Spirit bonus means you'll recharge your mana pool (and health) at a slightly faster rate then similarly equipped mages. Perception is also useful for spotting lurking creeps with stealth or an enemy Rogue. As low hitpoint casters, mages are often early targets of sneaky types.
The human bonus to mace and sword skills might not serve the mage very well, as mages can only use swords, and even then only with training. Since swinging with a weapon is usually your tertiary source of damage (after spells and wands), and staves tend to have more caster-oriented bonuses besides, you might not get much milage out of this bonus.
Horde
Troll
Troll abilities (with the exception of the thrown weapon bonus) all have their own individual upside and downside for a troll. Thrown weapons are unusable by mages, and as such this skill is wasted on the Troll Mage.
The troll regeneration ability means less time spent eating. Since mages summon free food and have their downtime stretched by mana, not health, this seems like a weak bonus at best. However, since you retain some health regeneration in combat, troll mages can take a hit or two more then other mages before keeling over.
Trolls have a love/hate relationship with their berzerk ability. A 25% casting speed bonus is significant, but the ability doesn't activate automatically; a troll needs to realize she's below 20% health and hit the berzerk button. Since they're usually worried about dying at that point, most trolls forget to go berzerk. Mages have a doubly hard time with this, since 20% mage health is very close to death. In addition, when in melee - a common cause of being low on health - a mage will be favoring instant spells (no casting time) anyway. However, on the rare instances when the planets align and a troll mage goes berzerk and lives to dish out some damage, the effect is startling.
The troll beast bonus has the same value for mages as any other class. There are a lot of beasts in the world, this will help you with them significantly.
Undead
Underwater Breathing and Shadow Resistance have obvious value, especially with quests that bring you to the bottom of a lake, or when fighting priests or warlocks. The Will of the Forsaken ability will break and fear, sleep or charm effects on you, and make you immune for the next 20 seconds, during which time whatever cast the effect on you will try to re-cast it, buying you time to peel off a spell. Since a mage's capacity for damage and their ability to escape persuit makes them prime targets for such crowd control spells, this is a useful ability.
Like the troll's regenerate ability, cannibalize isn't very useful. Mages that can summon themselves a snack and eat it while drinking to regenerate mana, cannabalizing is almost a waste of time for an undead caster.