Well, hold on, CM Thundgot has a few things to say on the subject:
Of course there are rules concerning loot. You won't find a level 10 random mob dropping the same loot as Illidan. Edwin VanCleef drops stuff no other mob drops - not even randomly from level 70 mobs outdoors. Some mobs can drop cloth, others not. Etc, etc. That does not mean the characters involved, the time of day, the guild, the raid leader, weather it's raining in Shanghai or the colour of your bicycle has any effect on the drop...
Whew. I feel better now that I don't have to keep tracking weather conditions in China to determine optimal raid times. But on a serious note, why aren't there more rules concerning loot? Before the Horde had Paladins and the Alliance had Shamans you didn't have Paladin set drops in a Horde group or Shaman set drops in an Alliance group. This random loot system can be kind of harsh -- why not add some more rules to the system to make it easier on players? Thundgot has some thoughts on this, too:
If you want that kind of predictability, why not program an equipment check and drop exactly what you need - every time? Got 7/8 of a set? Next run you will get that 8th piece, no matter what. Can you make Mageweave bandages? Let's not drop Silk Cloth, but only Mageweave. I honestly think that'd be boring, and prefer the random chances and excitement of hoping to be lucky. It's about finding a balance. You know where the item can drop, but you can't be sure it will drop.
I do see his point, but honestly, I don't think Thundgot was ever the unfortunate last person without tier 2 pants when his guild decided to stop running Molten Core to focus completely on Blackwing Lair and Ahn'Qiraj. However, I do think Blizzard has addressed some of these concerns -- raid dungeons after Molten Core have entire armor sets drop completely within that dungeon, so players completing higher level sets don't have that annoying problem of needing Ragnaros to finish off their tier 2 set that's otherwise in Blackwing Lair. And in Burning Crusade dungeons, bosses drop tokens which can be turned in by multiple classes for set items rather than dropping specific set items. The random loot system is still a headache, but Blizzard seems to be taking at least some steps towards making it a bit less of one.