Tobold blogs about his guild's raiding policy for Wrath. The more serious raiders feel that their progress is being slowed by "tourists" who aren't putting in the effort, and they're formally dividing the guild into raider and non-raider categories. As Tobold notes, this sort of thing tends to end poorly. Segregating out portions of the membership cuts the ties that hold the guild together in the first place.
This gets back to a question I discussed a month ago about how it is hard for the developers to match players up with like-minded comrades in game. As I discussed at the time, the developers don't have a huge amount of direct control over player recruitment practices. However, they do have indirect control through the kinds of content they choose to develop.
Leveling Guilds in a Massively Single Player Game
Because WoW players can solo to the cap level, there's no need for dedicated leveling guilds. Players CAN form guilds that simply consist of themselves and their friends for out of game reasons, but the in-game incentives for making a leveling guild (e.g. groups for leveling instances) are outweighed by the logistics. Players are going to level at different speeds and WoW does not have a good system for playing with characters of different levels. Also, most players seem to want to raid when they get to the cap, and a guild with 80% of its membership in the 20-60 bracket probably isn't going to be able to field raids.
Low end raiding and the 7-day lockout
Blizzard says that they want to encourage lower end raiding, and that they have tuned the revamped Naxx to provide a lower entry barrier. However, they're still keeping some of the major features that make raiding inflexible, notably the group lockout, because they don't have a good way of incentivizing the zone. If they make the loot from Naxx good and allow it to be zerg pugged like the old school 10-man Baron/Scholo/UBRS raids, players will skip the 5-mans to zerg Naxx. If they don't make the loot from Naxx good enough to justify the extra logistics, players won't take the time to run the dungeon. Given how slowly Blizzard adds new content to the game, they can't afford to spend time developing content players won't use, so Blizzard is pigeon-holed into the current raiding model.
Thing is, the issues with the 10-man, 7-day lockout system haven't changed since they caused so much turmoil at the launch of TBC. You can't reliably fill the group unless you have more than 10 players, but you can't put more than 10 players in the group. As a result, you're going to have a non-trivial portion of your guild (generally the excess DPS) sitting out content for the week, missing out on both rewards and the experience (player skill, not character exp) that would allow them to contribute in the future. This leads to the situation that Tobold is in now, where the more serious raiders accuse everyone else of holding up progress and demand various reforms before ultimately leaving for a more advanced guild.
What does the system favor?
At the end of the day, players will do what the incentives tell them to. You don't need a guild to level, and you don't really need a guild to do battlegrounds, so most guilds don't focus on these areas. You DO need a guild to raid, so guilds form to allow raiding. Once Blizzard gets guilds to stick their toes in the water, the riptide current of progression takes over and forces hard choices between friendship and achievement - no matter how good a player is, an equally skilled player with better gear and more money to spend on consumables contributes more to the raid.
In my experience, it's a very tough balance that proves very hard for guilds to manage. I've been in several guilds that tried to pull off so-called "casual raiding", and all have either disbanded or merged with a more serious raiding guild in order to pursue more challenging content. (My most recent raiding-optional guild went the latter route yesterday.) It's a difficult situation to be in, especially if you don't enjoy raiding and are limited to a single server due to owning way too many alts to be able to transfer elsewhere.
At the end of the day, you do still have the option NOT to do as the Romans (Azerothians, etc) do. You just might not have much company.