So I solo'd my Paladin as Retribution until 110, giving my choicest relic quest rewards to Truthguard while I kind of split my artifact power between it and Ashbringer. Then I hit 110, started doing Suramar, and decided to just be Protection all the time.
The good news is that this isn't such a bad idea - mobs weren't dying exceptionally fast when I was playing as Retribution (except when I got the rare Ashbringer hidden proc that insta-kills non-elite demons and undead and leaves a pile of ash) and when I switched to the tank spec, it wasn't like they were taking forever to die either (Shield of the Righteous hits like a truck.)
So after a brief period as Unholy on my Death Knight, I went Blood. After one zone of Havoc on my Demon Hunter, I went Vengeance.
We're still super-early in the expansion, which is why I don't know that I'd worry too much about this yet, but it really seems like you have to pick a spec and stick with it. This applies even to pure-DPS classes - there's a lot of investment I'll be putting into the Subtlety artifact on my Rogue and as cool as Outlaw looks now, at least for now I'm really going to have to either hope that artifact power gains increase exponentially or I'll have to bring up my other Rogue (though he's going to have to wait for about eight other characters to hit 110 first.)
Clearly they have anticipated that being stuck to a non-DPS spec if you want to perform a non-dps role means you'll need to have some decent power in order to solo. I assume healers similarly get powered-up when it comes to damage capabilities. As a tank I can pull very large numbers of enemies and survive (granted I'm playing three tank classes with self-healing built into their tanking strategy.)
What I worry about, though, is our future raiding prospects - in my guild there are only a couple of healing-capable characters at the level cap. Now that's clearly something that's going to improve with time, but I know that I'm Protection, and my real-life friend who plays is a Balance Druid. He's occasionally willing to heal, but how easy is it going to be to "occasionally" heal when your artifact has little to none of the traits you need?
I suppose the issue is how tuned the encounters are around artifact power. How much of an investment to they expect for you to have put into it? Clearly we're not going to be able to fill out our artifacts for a while (I'm just one intermediary trait away from getting my second major trait on Truthguard) so how is that going to balance out?
I'm also really eager to know more about artifact knowledge. It seems like it will help catch up (or really just help us fill things out at all,) but is the value of each level going to go up (right now it's a 25% bonus, but in a week or two will it go up to 50%?) or is it just going to take less and less time to research each level?
I'm a committed tank - I've been mainspec Protection since I figured out you should put your talent points in a single tree (so a couple months into playing) - but I do really want to be able to go out and use the Ashbringer some of the time.
We're just eight days into the expansion, though. I'll be keeping a close eye on this issue.