CRZ is an attempt to balance the load on the servers, but it's a poor solution to a problem that Blizzard themselves have caused through poor game design, and bad policies.
Zones are over or under utilised primarily due to either faction imbalance, which happens because Blizzard won't do anything to stop people transferring onto an already hugely overpopulated faction or realm (see PvP servers, with 9:1 or worse faction ratios), or due to level imbalance, which happens because they only rarely release new content for anything but endgame, the only time players are encouraged to play lower levels being when there's a new race or class, and then they'll level through old content as fast as they can to get to the endgame anyway.
Meanwhile GW2 limits transfers onto over populated realms, in order to prevent over/under population. Also by not having factions there's no way for players to try and join one in order to get an advantage over the other (and lets face it, while WoW may have factions, Bliz have done fuck all to make use of them for gameplay reasons, and they're only still there because of the story. World PvP is dead, and BGs could work just as well without factions, if not better thanks to racials no longer being a part of PvP imbalance).
GW2's dynamic deleveling means that new content can be spread out to cover the whole level range, and players won't have to roll a new character just to take advantage of low level expansion content. Also that game's overflow servers are a much better way to handle overpopulation, as they can handle the rush into starter or new endgame content that happens with a new expansion, much better than WoW's shunting players off into the underutilised parts of other realms (which end up being over utilised anyway, because of the sheer size of the influx of players that comes with an expansion).
One of the biggest complaints against CRZ is that it makes finding mining or herbing nodes, or just finding mobs to complete a quest, much, much harder, because of the number or players it puts into one place. Again, the way GW2 was developed took this problem into account. By matching the resource nodes to the number of players, exactly, by giving each player their own set of nodes, the resources will scale to match the number of players on the servers. Meanwhile in WoW, on everpopulated realms resources become scarce to the point that crafting professions cannot get materials, except at vastly inflated prices, because the limited number of nodes are shared between all the players.
All this is just a small part of the reason I only play WoW as F2P, that reason being that I'm no longer going to pay a subscriptions for a game that isn't being developed for the benefit of those who are paying for it.
