Like anything else, different people play the game for different reasons, will have different experiences, and will be affected differently. For example, I play WoW largely as a solitaire endeavor, and if I do join a guild it's mostly just to chat with people. I don't tend to get very connected with people in MMOs. I've brought two characters to level 70 almost exclusively through solo play.
For a while I played around with an RP guild called the Academy of the Arcane. It was structured as a "school," complete with "classes" (basically, meeting you could RSVP for where a senior member of the guild would give people tips on playing their class, in-character). That was an interesting experience. I've been a part of a guild called Ordo Serpensis which was largely just a social guild, and I didn't do very much with them. I've been a part of a raiding guild called Infectious, as I was trying to get more into the game and gear up for raiding. It didn't really work out, once that character hit 70 I just got bored with it.
Anyways, I think the potentially most interesting and unique-to-the-medium experience of the three would be the raiding guild, so that might be worth exploring.
As I'm sure you know, classes fulfill roles. The three main roles are DPS, heals and tanking. Some classes are well-suited to one role, though are not restricted to it (e.g. in WoW, warriors are good defenders, but can DPS; priests are great healers and can also DPS). Other classes, though, are capable of filling two or even all three roles. These classes are referred to as hybrid classes. Druids, for example, are the quintessential hybrid class, and depending on your spec and gear they can be exemplary tanks, healers or DPS.
I've never been on an actual raid, so most of my info comes from 5-mans (smaller dungeons) and what I've read. The tank is the keystone of any group, no matter the size, and can also be one of the more difficult and stressful jobs. If the tank fails, unless whatever you're fighting is almost dead, it's almost assuredly going to result in a TPK. For this reason, there's a lot of pressure put on the tank.
There's a support role called an off-tank. This is someone whose job it is to take some of the heat off the MT (main tank). The off-tank might be responsible for keeping some minor foes busy while the group focuses on a more dangerous one, or might be responsible for grabbing "adds."
Another important pseudo-role is CC (crowd control). Many classes have effects which can effectively remove a monster from a fight for a limited time; mages can polymorph enemies into sheep, rogues can sap enemies, hunters can lay a trap to freeze enemies, etc.
Some terminology:
To "con" something is to gauge its level relative to your own. Short for consider. Sometimes this is explicit (as in WoW, where although a mob might con yellow (meaning it's slightly above your level), you can also see its exact level), and sometimes it is not (e.g. in EverQuest, it just tells you that something looks tougher or weaker, with rough degrees). Conning is usually done on a color system; red and purples are often very tough, yellows are usually slightly above your level and should be easy enough. Whites are about the same level as you. Blues and greens are often lower level, and grays are so low level as to be trivial.
A "mob" is an enemy. Usually used as a single enemy. I believe it stems from MUDs, where it was short for "mobile."
A "pull" (noun) is an enemy or a group of enemies who can be pulled in a discrete unit.
To "pull" (verb) is to grab a pull. This is typically done with a ranged attack, and frequently by someone other than the tank. If the situation allows, it's usually preferable to allow the tank to make the pull, even if it means running up to them and aggroing them.
"Aggro" (a.k.a. "Threat") (noun) is a measure of how much of a target you are to a mob. Aggro is mostly built up in three ways: damaging, healing, and taunting. Tanks usually employ a method of taunting an enemy to add "virtual damage" to their threat, and make them appear more threatening to the mob, causing it to stick to them. This is not always an actual taunt; for example, in WoW a paladin has an ability which increases the threat generated by their holy damage, so if they do 100 holy damage, it might cause as much threat as 150 damage.
"Aggro" (verb) is to gain the attention of a mob. The tank wants to get and keep aggro; the others want to avoid it like the plague in most cases.
"Spec" is short for specification or specialization; in a nutshell, it's the role you've tuned your character to play. For example, a paladin in WoW might spec for tanking or healing. A player might also "respec," which means to change the way their character is specced. This might be a minor change, perhaps just juggling a few suboptimal choices to more efficient ones and maintaining your chosen role, or it might be a major change so that you can take on an entirely new role or strategy.
"Buffs" are persistent enchantments which increase a player's or mob's abilities. Debuffs are the opposite (e.g. poisons, curses)
"HoT" and "DoT" are Heal- and Damage-over-Time. These are kind of like a crosses between a normal attack or heal and a buff or debuff; they tend to have high efficiency from a cost stand point (e.g. high damage-to-mana ratio) but take time to deliver their full payload.
A "tick" is every time a HoT or DoT happens. For example, a HoT might heal 100 damage every 3 seconds for 15 seconds. This means that it has 5 ticks, and each tick is 100 health.
To "Proc" is for a special ability to trigger, usually (but not always) an ability tied to a piece of equipment rather than an innate ability of the character. For example, a sword might have a random chance to cause a fiery explosion for extra damage, or to freeze an enemy. When it does so, you would say it has procced.
Every MMO has its own jargon, but one common element is the ever-prevalent acronym. This is especially true for names for places. Any WoW player who's played long enough will know what BRD, LBRS and UBRS mean instinctively (they're all dungeons-- blackrock depths, lower blackrock spire, upper blackrock spire). In City of Heroes, the biggest, baddest mobs are AVs (arch villains). Every MMO will have a rich-- and unique-- lexicon of acronyms. There are of course acronyms common to many MMOs, such as the above HoT and DoT.
I'm sure I'm leaving a lot out, but I hope that's a good start.
