Aggro radius

The aggro radius is the distance from a player at which a mob will stop its normal behavior and engage the player in combat (see aggro).

Additional notes

 * The aggro radius of a mob will vary depending on the difference between the level of the player character and the mob. The higher the level of the character than the mob, the smaller the aggro radius.
 * Aggro radius also applies mob to mob. A mob with hate passing through another mob's aggro radius will pass on hate or threat to the new mob. Thus the new mob will join the party and add to the combat.
 * This only applies to mobs of a similar type. A bloodsail pirate will co-aggro another bloodsail pirate within his aggro radius, but a bloodsail pirate will not co-aggro a gorilla or vice-versa.
 * This applies even if the mobs would not normally aggro when their attacker stepped within their aggro radius (i.e. mobs whose identifying text isn't red). If your reputation with the Gelkis centaurs is merely Unfriendly, so that their text appears orange instead of red, attacking one will cause other Gelkis centaurs standing next to it to attack you also.
 * When a lower level character follows a higher level character, they should keep in mind that the higher level character has a smaller aggro radius and may run by a mob safely that will be attracted by the larger aggro radius of the lower level character.
 * Also, higher level characters should take care when running by mobs with lower level characters following them (for the same reasons stated above about lower level characters).

Formula
Current in-game testing shows the following facts:
 * The aggro radius of a mob having the same level as the player is roughly 20 yards (~ 18 meters)
 * Aggro radius varies with level difference at a rate of roughly 1 yard/level.
 * Minimum aggro radius for a mob seems to be combat range (5 yards).
 * A Hunter's pet seems to ignore the minimum aggro radius for low level mobs, even when controlled through Eyes of the Beast.
 * Aggro radius is capped at a maximum of 45 yards, that is, when the mob is 25 or more levels higher than the player.
 * Vertical distance seems to be weighted much more heavily than horizontal distance. A mob standing on a ledge 4 yards above your head can fail to aggro even if standing 4 yards away from the same mob on level ground will draw aggro.

Speculation

 * Some mobs appear to have higher aggro radius against stealthed players. Examples would be Gordok Mastiffs in Dire Maul (north), and the bloodhounds in Blackrock Depths. This may be a matter of exceptionally good stealth detection rather than an increased aggro radius, however. --Confirmed to be stealth detection now that stealth detecting mobs have a symbol above their head.
 * Some mobs appear to have higher aggro radius than expected for their level against all players. The Warpwood Crusher in Dire Maul (east) has a much higher than normal aggro radius.  Against level 70 players, their aggro radius seems to be reduced as would be expected when compared to a level 60 player, but the aggro radius is still far larger than would be expected for a typical level 56 elite mob.  Casual testing estimates 1.5-2 times the normal aggro radius of a similar level mob.
 * The aggro radius around a mob does not appear to be symmetrical: The distance at which a player will draw aggro while standing in front of a mob tends to be greater than the distance at which the same player will draw aggro while standing behind the mob. This may, however, be because the mob is often walking forward, and the location of the mob reported to the WoW client will lag a fraction of a second behind its actual location on the realm server.  It may also be because the actual location for a mob may be forward of the center of its body (e.g. the "hit box" for a wolf is centered on its head, not on the center of the red targetting disc). Chances are however that this is intended behaviour, and in-game situations suggest this is a real effect not (only) caused by the mobs moving forward.
 * The assist radius of a mob (the distance within which another mob friendly to the aggroed mob will also aggro) seems to be different from the aggro radius in many cases.
 * Mechanical mobs in general (though not always) seem to have a slightly higher aggro radius than other mobs their level. The mechanical golems in Westfall, for example, seem to be much more aggressive than normal.
 * Mobs with the word "starving" in their name (Starving Dire Wolves in Duskwood, Starving Blisterpaws in Tanaris, etc.) also seem to have a higher aggro radius than other mobs, presumably because they are starving. The Starving Dire Wolves (levels 19-20) appear to aggro from further away than the nearby Rabid Dire Wolves (levels 20-21), the Starving Blisterpaws appear to aggro from further away than the nearby similarly leveled buzzard and scorpid mobs, and so on.  Exercise caution around starving mobs.

False speculations

 * Some people say that different classes have different aggro radius against different types of creatures (e.g. Hunters can come closer to Beasts without aggroing than other classes). This is false, though it is based on truth. There was a period in early beta during which this was the case.