Enough preamble, let's dig into the tracks. (Youtube heavy!)
The opening never took me as especially striking. The intro cinematic introduced six of the races and the music jumped between their themes. Warcraft III cinematics surpassed it in scope. The racial motifs are present, most notable when the tauren appear. The music is very much tied to the action though. It's serviceable but no more.
Each capital had its own track in the game. They consisted of an intro, played at the entrance to a city and several 'about town' subtracks played across the various districts. there's a certain amount of cohesion between them. All the intros have a bombastic beginning, echoing the sense of scale you get when entering one of the cities. They each run with two main instruments, typically horns and drums. Ironforge and Stormwind have limited similarity, with Ironforge coming across as military while Stormwind is more religious. Orgrimmar and Undercity share similar timbres, with Undercity being a more sinister play while Orgrimmar emphasises the heavy drums for a warrior's beat. This leaves Darnassus and my personal favourite, Thunder Bluff. Both are Kalimdor natives, the wilder side of the world. While Darnassus imitates the grandeur of Stormwind however Thunder Bluff goes for a far gentler sound. Its intro is the most chilled out of the six, with the backing drum being a steady beat that reassures. It carries that theme on from the intro, developing it without the beat into three distinct subtracks that see play throughout tauren settlements.
In terms of zone soundtracks there's plenty to choose from, roughly grouped by the terrain type to which they're applied. The desert themes that see play in Tanaris and Silithus are piping flutes that speak of the hissing sand. Booty Bay bears the thick jungle theme that also sees use in Feralas. The plains theme will be burned into your head after the Barrens and Westfall. However it's the track titled Soggy Place that wins out for me. This track sees use in many caves and dungeons because of its brooding tone with long foghorn blasts against a foreboding drumset. In terms of zones it sees use in Un'goro, Wetlands, Swamp of Sorrows and best of all, Dustwallow Marsh. These places all tended to spring monsters on you from unseen angles with shallow pools and heavy tree cover. The track is split into several variations, each with their own arrangement of the primary and secondary instruments. This very simple method was used on many of the zone tracks and allowed for variation while keeping the track as a whole, cohesive.
The original Ahn'Qiraj soundtrack develops on the sounds originally explored in the Desert track but develops them further. Desert hints at a sleeping evil, Ahn'Qiraj heralds its awakening. It actually has two tracks, one for each of the raid encounters. Each has a distinct intro with Exterior favouring drums while Interior went for pipes. The both have quite an eerie feel to them, a sense of wrongness played on in the exaggerated echoing of the notes. It's not hard to spot the inpsiration. Ahn'Qiraj is the homebase of a Cthulhu inspired deity and Lovecraft described one such, Azathoth as "amidst the muffled, maddening beating of vile drums and the thin monotonous whine of accursed flutes"
However it's not all good. These tracks did manage to clash with where they were set sometimes. Raid instances were tricky in that they're all action with little scope for ambience absorption due to the 20 to 40 people there to get the bosses down. Exterior track gets rather conventional after its opening. It could have benefited from more environmental cues like Soggy Place. Interior suffers from having to cover a lot of ground, so much that there were actually special mounts in the Ahn'Qiraj interior to allow you to get through it faster. As such the track tended to cause listener fatigue.
Before we go. Here's a some great use of one of the mini-tracks.