I am going to ramble so hard, I'll pretty much forget whatever point I'm trying to make about half way through.
This is precisely how I feel about black metal vocals. Except for classic Opeth, I pretty much hate all black metal vocals, no matter how I constantly try to sample additional bands and songs. But I keep trying. Vocals alone are one of my biggest hindrances to beefing up my library.
As for Decapitated, one of the reasons I like these latest three albums so much is their vocalist. I know death metal vocals aren't everybody's cup of tea, but this guy pushes all the right buttons for me.
Man, I wish I could say. I get really, really hung up on vocalists, and one of my biggest problems with modern clean singers is that they all tend to sound depressed or whiny, living in some sort of perpetual hipster victimhood.
There's a trend for bands modern bands to switch between clean and harsh vocals throughout a song, and I pretty much hate all of it. Not because I hate the genre or the act of switching back and forth, but because the clean vocals are just so fucking lame. Now, I know switching back and forth between styles isn't a new thing, but it wasn't quite as popular when Fear Factory was doing it 20-25 years ago compared to the sudden proliferation of the style today. And the reason I bring this up is merely to point out the attitude differences between Fear Factory's classic clean vocals (another), and what just sounds like melodramatic whining to me from so many other bands today.
Anyway, to more directly answer your question (that previous paragraph is like 2% of a tangent I've been trying to keep inside a while), maybe trying listening to bands that mix it up a bit. Opeth, Fear Factory, Iced Earth, many of their songs swing back and forth on the harsh/clean vocal scale (some more than others), and they can help ease you into music that tends to swing one way and stay that way for longer periods of time.
I don't know. I know I'm giving a horrible reply to the question, but suffice to say that I feel like, with a lot of these bands, newer listeners just have to keep working at different ones till something finally clicks. And when it does click, you'll never understand what sounded wrong in the first place.
I don't know everything you're familiar with, but here's a couple sample songs with technically impressive music, but cleaner vocals you may or may not like. I don't know much about Power Metal, I got into Iced Earth during The Dark Saga (a Spawn comic book concept album), and to me they've become more and more like a weird hybrid of Judas Priest and Iron Maiden ever since.