We like random loot being random, as long as it isn’t so frustratingly random that you stop enjoying the experience.
Actually this is my biggest problem with random loot in World of Warcraft. I ran Forge of Souls 34 times on my Warrior tank to get that one damn tanking plate belt off of the Devourer of Souls. Do you know how often it dropped? 0 times. NONE. I don't even mean it dropped and a dps death knight stole it from me, I mean it didn't drop at all.
Similar story with the STR DPS trinket off of Archbishop Benedictus in Hour of Twilight. Think I ran it on my dps Warrior about 20 times. How often did that STR trinket drop? -0- times.
I can't even count the amount of times I ran Trial of the Champion for IL200 items like the trinkets I needed only for them to either never drop, or drop after 30 or 40 runs. I find the random loot system to be so crazy that I find it broken and EXTREMELY frustrating and unfun. I just do not think that items that literally NO ONE in the party can use should even drop off the boss.
And don't even get me started on how long I raided 25-man Trial of the Crusader to get the Belt of the Merciless Killer to complete my Tier 9 look. I NEVER got it, and then Cataclysm launched and I couldn't find anyone to ever do TOC 25 with me ever again on my Rogue.
While I agree that everyone getting the exact items they want each time would be a little ridiculous and kill dungeoneering and raiding because people wouldn't have to do it for very long, I find that the system in place right now is the other end of the extreme.
What I'd like to see with the loot system is either for everything to be purchasable with Justice/Valor (so you can get everything you want eventually), or for there to be some kind of mechanic that -counts- the amount of times you've killed a boss or cleared content and not gotten an item you can use that then makes it more likely that you'll get an item the more times you haven't gotten it.
Basically a streak-breaker mechanic like the one implemented for quest item drops.