I generally find clean vocals irritating when they're done real over the top as seems to be the case in most traditional/power metal. I can handle stuff like Winds or Type O Negative easily and even the crazy vocals Simen Hestnaes busts out in various bands, its really only when they are so high pitched you might as well have a woman singing that it gets irritating. I haven't heard enough of the ole traditional bands to be any kind of a judge, but about 90% of the time a song like "Run to the Hills" or something like that is going to get on my nerves rather than bring me joy. Maybe that's a bad example, but hey I'm inexperienced.
Vintersorg and Vortex rule, but when you start getting something that sounds like Hammerfall, I'm gonna turn and go my seperate way. I also saw a Mercyful Fate video which is a band I think is hailed around here. Music wasn't bad, vocals made me change the channel. Besides, there is something too overwhelmingly evil and bad ass about growled vocals that I just can't deny. Plus they sound better almost regardless of the situation.