I'm seeing this play out on a progression server (Phinny) which I thought the instancing would help with, but really it may not at all:
PoP is pretty much all good for the hardcore focused raiding guild. On progression, with the increases in player power, known strats and general raid force experience this content is like brushing teeth for many. Phinny's rules mean more of these guilds are succeeding on a single server than ever before. There is no real issues for this niche playstyle (Fippy server may be the only server where more paid accounts are hardcore raiders than not... as the hardcore raid guild is pretty much all that is left there) outside of some of the keying decisions the devs have made.
However, PoP may now be even worse of a community killer than it was originally. Originally most servers had a top guild or two blocking everyone else. So everyone else had to either join them (if you could), or stick together and do other things. Some raid-focused people got discouraged by this for sure. Lots of guilds weathered this (though it was a straw in the pile that GoD and WoW eventually broke).
On Phinny, it seems that any guild with a mixed playerbase (there's quite a few with several hundred "casuals" and a raid core of 60+ members) is feeling this the worst. Here's what seems to be happening:
-phinny has a leveling culture of only using a few zones. Not ideal, but at least you could mix together and get groups when they were open to all zones like Velks.
-many will tell you the xp rate is fine, but they use the juice (xp pots), keep their group level ranges extremely tight, and only use those few zones. These people also tend to play a lot. Some of these people are already saying that PoP progression is going too fast and next expac needs to be sooner. This is a loud group on the official boards, but one that misses the point of progression servers for anyone not in-era hardcore raiding (a very large population on Phinny, that PoP may begin to deflate).
-the few zones preferred in PoP are mostly partly behind keying (or are stupidly busy with people keying up including those that like to KS and train), so now the mixed guild has the raider core leveling in PoP and a large chunk of the guild left out. This hasn't hit critical mass yet as long as a person is over 60, but that will happen soon (when all the 65 raiders are only in elemental planes).
-worst hit is the person slightly off in level that could in theory group with a level 60 (and now 65). This was very common among friends back in the day, aside from "I am PLing you" (usually for pay) on Phinny it is rare... with some irony being that 50s leveling zones like GE are being taken over by level 62s. Exception at times to this is the cleric...
-obviously part of this is the hardcore raiders are pushing their mains right now
So in the aforementioned "mixed guild" we now see the raiders spending all their leveling time together as well. This doesn't harm the raid core directly, but down the line it hurts the overall guild (if mixed or family is truly the vision) and server community. Pretty much none of this applies to a focused-vision raid guild. But if you are a leader/officer in any sort of mixed guild it bears some thought.