there's a lot of community on it. If you're on a pvp server you will be killing a lot of opposing people (and being killed--"ganking" refers to when a higher-level player kills a lower level player that has no chance of defending themselves). If you're on a normal server, you get a choice of when to do that. My brother and I were leveling our characters today, running different dungeons, pvping outside Scarlet Monastery, doing daily quests in Outlands. I have 1,500 gold and need 3,500 more to get an epic flying mount, so I do a lot of dailies and farming. I need to start pvping more to get that new armor, since mine sucks (and paladins just suck in general).
As for $$, it's whatever the cost of buying the disks are (he'd have to buy both WoW, and The Burning Crusade expansion, and the Wrath of the Lich King expansion is coming out soon but there's no set day). And then its about $15 a month, give or take payment plan.
As for other charges, just make sure your teen doesn't fall for the gold spammers. Just like in your email, companies will create a character, send them into the main hubs in WoW, and spam out a website to visit to get free gold, or an offer to level your character. Make sure he doesn't touch those--they ask for your sign-in name and password, and if they have that they can get your credit card information off the worldofwarcraft.com site.
(ETA: As for wanting to buy gold from those who actually *do* sell you gold, it's against the rules and you can have your account canceled if caught.)
As for time consuming, quests could take a few minutes to hours (but you can always stop in the middle). Battlegrounds (scheduled pvp) can last a few minutes to an hour or more, if the opposing groups are at a stalemate. Dungeons can take an hour or five, depending on who is your group or how many times you die. Raids take hours. Battlegrounds, dungeons, and raids, you can't quit unless the whole group does (well, you can quit, but your group will get pissed at you, especially if you're a tank or healer or dps or crowd control--basically, anyone).
As for add-ons, there are a few safe-to-download things. Cosmos UI adds things like coordinates for finding mobs, damage meters, drop rates, etc, and is safe to use. The alternate to Cosmos is Titan, but Cosmos is more common. Your son might also download ventrilo (a lot of guilds have designated ventrilo servers), which is a system to verbally speak to other players (WoW added a microphone/speaker system, but I've never gotten it to cooperate).
(ETA: as for cursing, communication is through a chat window with different channels, like Trade channel or General channel, which your son can see. There is an option to sensor bad words, and you get the traditional &*#$^ symbols. There is supposed to be a live talk option, but my brother and I can never get it to work (I'm on mac, he's on dell). Vent is live talk, and there's no sensoring because it is a separate entity from WoW, and the channels in vent are privately owned and controlled by whoever pays for it.)
And as for playing together, two people cannot play on one account at the same time. I think it's against rules to even share an account, but I'm not sure. My brother has his account and I have mine so that we can play at the same time.
Basically, it's like crack.