I will edit and update this guide accordingly as suggestions are shared.
Disclaimer: This is as much a guide as it is a discussion, and the guide itself is a take on different builds and styles of how one may be able to play as an Assassination Rogue.
There are at least three factors one might consider when choosing a race: appearance, utility, and damage.
Every Man for Himself (Instant, 2 min cooldown): Removes all movement impairing effects and all effects which cause loss of control of your character. This effect shares a cooldown with other similar effects.
Since Every Man for Himself shares cooldown with Cataclysmic Gladiator's Medallion of Tenacity, Humans may replace their Medallions with a damage increasing trinket. Players who raid or use the Looking For Raid system may take full advantage of this racial, but if not, the racial is convenient at best.
Stoneform (Instant, 2 min cooldown): Removes all poison, disease and bleed effects and reduces all damage taken by 10% for 8 sec.
While the 10% reduced damage taken does indeed have its uses, being able to remove bleed effects (which is the only DoT a Rogue can't remove) to re-enter stealth will give you a distinct advantage when playing. Furthermore, this ability could save you from having to use your Cloak of Shadows ability if the only DoTs on you are poisons, diseases, and/or bleeds.
Elusiveness: Reduces the chance enemies have to detect you while Shadowmelded and increases your speed while stealthed by 5%.
Quickness: Reduces the chance that melee and ranged attackers will hit you by 2%.
Shadowmeld (Instant, 2 min cooldown): Activate to slip into the shadows, reducing the chance for enemies to detect your presence. Lasts until cancelled or upon moving. Any threat is restored versus enemies still in combat upon cancellation of this effect.
Elusiveness is somewhat insignificant, but not completely without merit. Quickness is only advantageous if whoever is attacking you doesn't have 7% hit; some do, and some don't. Shadowmeld is a fantastic ability for Assassination Rogues in particular because they don't have Preparation; using Shadowmeld removes you from combat, and therefore you can re-enter stealth, so it may act as a second Vanish. If you use Shadowmeld, you can "eat" a spell and/or ranged attack if timed correctly, and any enemy player who has you set as a Focus Target will have it cleared once you use Shadowmeld.
Escape Artist (Instant, 1.5 min cooldown): Escape the effects of any immobilization or movement speed reduction effect.
Shortblade Specialization (Requires One-Handed Swords, Daggers): Expertise with Daggers and One-Handed Swords increased by 3.
Shortblade Specialization is a solid racial, it can be very helpful if you choose to reach 5% expertise (amount necessary to bypass a caster's dodge). Escape Artist is very helpful for Assassination Rogues since their mobility is very lackluster, plus it allows them to save their Trinket cooldown for Stuns, Blinds, or other forms of crowd control rather than waste it on a root or slow.
Darkflight (Instant, 2 min cooldown): Activates your true form, increasing current movement speed by an additional 40% for 10 sec.
Viciousness: Increases critical strike chance by 1%.
Viciousness is of hardly any use, and given the fact that Rogues have a 1 minute Sprint, Darkflight is lackluster at best; Gnome's Escape Artist would be better for mobility than a Worgen's Darkflight.
Conclusion of Alliance Races:
Human = two damage increasing trinkets.
Dwarf = can ensure re-stealth.
Night Elf = second vanish.
Gnome = improved mobility.
Worgen = a mediocre sprint.
In terms of utility, I rate them as follows:
Night Elf > Dwarf ≥ Gnome > Worgen > Human
In terms of damage, I rate them as follows:
Human > Gnome > Worgen > Night Elf > Dwarf