1. Creativity: It's an amazing challenge to try and roleplay everything your character does. There are lots of things that even the most hardcore of roleplayers tend to just overlook, such as the raid symbols I mentioned -- Malebrignon over at the Twisting Nether Gazette roleplays them by saying he has enchanted gems which can create various illusions his team can see to mark out targets and make plans for battle. Really all he's doing is changing the terminology he uses to talk about raid symbols, but he talks about it in a way that makes at least a little bit of sense within the context of the fantasy environment, and more importantly, reveals a little something about his character as well. "Raid symbols" sounds like gaming language, but "illusion gems" sounds like it could be part of a story.
Another player named Zima added a really nice touch of immersion in his guild's Ventrillo channel when he gave an inspirational speech before his guild attempted to kill Muru. Normally we think of raiding and voice chat as two of the least RP-friendly environments in WoW, and yet Zima makes it work pretty well for him, with a funny dwarven accent and a well-timed musical soundtrack in the background.
This sort of thing makes me wonder if there's anything in the game that can't be roleplayed with a bit of creativity.
2. Immersiveness: Another aspect to a game like this is the feeling that you are in another world, with its own rules and conditions that you enjoy. If the people around you are doing it well, this sort of roleplaying heightens the sense that you're taking part in a story instead of just playing a game. A lot of the regular noise you get in everyday conversation is filtered out, and people only talk about the things that apply to the situation and characters at hand.
In fact, I suspect this is one of the major draws of this kind of roleplaying -- it has the power to shut out so much of the noise overload we get from living in an information and advertisement-heavy society. Every day we are absolutely deluged with a spamload of words and images we don't want, and didn't ask for. Anything from the sounds of cars driving by on the road outside, to the sight of people kissing in public spaces, to the huge amount of time given to advertisements on television. When people play WoW on full-roleplay mode, all that stuff is shut out, so they can just concentrate on the game and the characters in it for a while.
1. Blandness: It's tough being creative all the time. Sometimes people don't have the energy to really get into it, and they just substitute their new, supposedly "in-character" phrases for things in the game, without really roleplaying them at all. So, for example, instead of saying "bye everybody, I have to log off," they might say, "bye everybody, I'm putting my hearthstone down." It means basically the same thing for guilds that roleplay hearthstones as a kind of magical communicator system which can explain things like guild chat, long-distance whispers, and other such forms of communication that wouldn't otherwise fit in a fantasy setting.
The problem isn't that people use these terminologies -- it's that the terminologies themselves aren't enough. Their purpose is to prevent issues like guild chat and logging off from disrupting the roleplaying atmosphere people like to create. "LF1M MrT need tank!" in the guild channel may seem like just so much useless noise to a roleplayer leveling up her new mage alt, but "All we need is a protector and we'll be ready to go to Magister's Terrace!" can feel the same way if that's the only sort of communication people are doing over the guild channel.
There needs to be a lot of real character and expression, relationships and real communication in a roleplaying guild in order to make this full-immersion feel genuine and fun.
2. Communication lockups: There are often some things you want to say or questions you want to ask as a player, even though your character would never do so. I've recently been very disappointed with the Jewelcrafting profession for example, and even though I originally created my draenei hunter thinking that she would be really into gems and jewels of all sorts, using them in a semi-religious way (like crystal balls or tarot cards) in addition to putting them in sockets. It would be a fun thing to roleplay if I didn't find the actual gaming element so dull and uninteresting. I wanted to ask other people in my guild if they thought it would be a good idea to drop it for engineering or something, or if maybe there was something really fun about Jewelcrafting that I just wasn't getting. I couldn't very well say "I'm thinking of dropping Jewelcrafting" one day and then "Look into this Living Ruby to see your future!" the next -- it just didn't make sense to me from my character's point of view.
Personally, I'm finding that I prefer the system that my blood elf warlock's guild uses. Their guild chat is in-character, but they have a special out-of-character channel that everyone in the guild can join. People use both all the time as much as they like, and the different colors you can assign to each channel help very much to distinguish them. If you log in and feel like being really creative, you can roleplay your heart out in guild chat, but if you're just tired and want to level for a bit, or ask about the best talent build without having to dance around gamer terminology, that out-of-character channel is right there for you.
In the end, however, it's all up to you. The whole reason different guilds exist is for people with similar wants and needs to get together and play the game the way they enjoy.