I saw that it said somewhere that you advance based on how well your realm is doing, not you personally. I don't really understand what that means, however, and would like an explanation.
At a high enough level, it makes some sense. If you and some others are trying to capture a keep, you don't advance for killing several opponents in a failed effort. You advance for actually capturing the keep. You still advance if your role was purely healing your teammates. Or purely crafting gear for them. The goal is to make it so that your incentives are to help your side do well, not to steal kills from your allies or run off and kill enemies in situations where it doesn't matter.
But that leaves two major categories of complications. First, who in the realm gets credit for what happened? I can understand giving equal credit to everyone who is actively playing the game. But does that mean that someone who plays for 15 minutes per day will advance just as fast as someone who plays for six hours per day? How about people who haven't logged in in a month? If the entire realm advances at the same rate, what about characters who are created later? Do they start out with a bunch of advancement, or are they way behind for a while?
Trying to tie it to players being active recently creates other issues. Is it that the account is active, or the particular character? What about someone who is logged in but AFK? Or perhaps auto-running into a wall while AFK? Or a simple macro to click randomly now and then without actually doing anything? That seems like it shouldn't help you advance.
If activity for personal advancement is based on time since last login, then you create incentives for people to just log in and out briefly if they weren't going to play that day. Having to spend 15 minutes doing that on every single alt each day before playing the one that you want to actually play would be especially stupid.
The other major category is, how do you prevent a runaway effect where one realm completely dominates? I realize that the hope of having three factions is that if one is somewhat stronger than the others, but not as strong as the other two combined, you create incentives for the other two to gang up on the strongest one. But what if a server is completely dominated by one realm? Join that realm and you'll advance much faster than if you join the others, in addition to dominating whatever actual combat happens. That's the nightmare scenario that can ruin a game, and realm-based advancement can make it more likely to happen.
I understand that the game is supposed to be heavy on horizontal progression, not vertical. The problems above surely aren't nearly as severe as they would be in a game heavy on vertical progression. But mostly horizontal progression isn't purely horizontal, and horizontal progression isn't no progression. If they're putting progression into the game at all, it's because they want players to care about it, and players will act accordingly.