So I'm pretty new to WoW, and ended up meeting mostly Horde-supporters. However, as I played and learned more, something seemed a bit off, so I did research.
I've done extensive study of the fundamental properties of the WoW universe: light, void, and twisting nether. I studied their relation to mysterious dieties (such as Elune and Earth Mother), titans, old gods, and void lords. I studied the events of WoW history in relation to different characters affiliations with these forces and beings and observed what purposes and reasoning they espoused and what effect the ultimately had on WoW history, whether beneficial or detrimental to the common convictions and aims of every society.
I concluded that the Alliance is most often rightly affiliated with the underlying forces which establish and uphold the common good, although they may personally abuse the good once it is established. The Horde often have rightly ordered desire for common good, yet more often try to achieve it by affiliation with forces most often understood as common evil, most often through ignorance but unfortunately sometimes exalted to positions of power despite full knowledge of foul affiliation. Alliance members can be corrupted as well, but are usually then turned upon.
Light may be a power agnostic in itself, but its affiliations increase the likelihood of either the maintenance of the common good in everyday function of life, or required capacity to uphold and defend the good against corruption and assaults. Arcane, void, and nether power sources likewise seem possible to wield without affiliation, yet with an increased concentration of utilization there is an increased likelihood of gaining the attention of evil forces that will corrupt or assault life itself.
Therefore, while I would say Alliance is overall the better pick for upholding the continued stability of life that all desire, this does not prevent persons within it from being jerks in themselves without affiliations with any great underlying forces or affiliations. Furthermore, the potential to be corrupted seems to be greatly increased when extreme sources of power are cultivated or when power is sought through arcane, void, or nether sources. The most common fault of Alliance heroes seems to most often manifest in a tricky Catch-22 where mercy for those with dark affiliations allows great horrors to manifest, yet to vanquish such affiliated persons with too great a disdain leads to being corrupted oneself.
Still, Horde is a much riskier affiliation, especially when their present warchief is so deeply wrapped up in underworld forces and having been appointed by religious convictions of one troll who worships seeming underworld forces that do not seem quite as trustworthy as, say, nature spirits worshiped by the Tauren and Night Elves. Being a paladin or druid in the Horde would seem to be in essential conflict with political affiliations that keeping a united Horde requires, much less the greater personal conflict of needing to approve/assist many activities that the Horde takes on and powers that it taps into in order to fight the Alliance. No one wants to be exterminated in war, yet one can't be short-sighted in the aims of preserving their life with respect to historically demonstrated essential universal dangers, despite ways that the Alliance can be jerks.
Due to this conflict of interests and looking at the long history of events, I wouldn't have much hope in being a good Horde member like a Tauren who would succeed in turning it around and away from dark affiliations. It makes more sense to be a Draenei or Elune-serving Night Elf if desiring to serve good with utmost personal integrity to ones aims. Alternatively, a Human paladin would be less "aware" in terms of the "reasons behind it all" yet would be in best position to influence the Alliance to not socially be jerks with the power and stability their rule has been bestowed/entrusted with by the light. It seems more possible than bettering the Horde since Alliance evils more often happen by personal fault than subversion by powerful cosmic beings.
Granted, a lot of people won't get that far into it or may choose just because they resonate with a particular race or maybe like to go ham and fully support evil, but I think many may also choose based on the society that they enjoy and later on after learning some history, have to do some soul-searching about whether what they are involved with is really working to secure what they care about. I think the Mag'har Orc are making a nice opening to explore that aspect of things, especially in interaction with the Tauren. Blizzard also seems to be making room for intentional self-contradiction, like if you wanted to be a Demonology Void Elf fighting for the light.