The main difference with the basement isn't so much the mobs mezzing (which is a pita if you have one pop on your group and your tank gets mezzed)
its the no safe camp for the shardtender room, when i take groups down there ( i LOVE the xp down there, i still haven't found anything comparable to a good dps group chaining the basement) we generally head to the left as soon as you enter the room and sit behind the girplan in the corner by the stairs against the wall. As long as you stay on the wall and fight against the wall, you don't get adds (except for the girplan repop its about 18 mins +-30 seconds)
Every time i've gone down there with a pickup group of random people, we've always ended up wiping on the pop of the local mob at some point. Guild/friend groups are never a problem as long as they stay on the ball. It isn't a light hearted camp like any of the upstairs camps where you can pull for hours on end and never had an add pop on you.
The huge bonus is.... the number of mobs within easy pulling distance, with a good group, you can keep down the shardmender (ac aug/caster robe), cyrec the arcane (runes and crap), frostkeeper rybah (runes and crap), mistwalker (drops a shield... and crap/runes afaik), shadowstalker (bit of a fun pull without clearing stuff -- drops a shield/runes/trib items) discordling surveyor? i think its the surveyor (crap belt/runes/lute dropper - which wanders around everything from Rybahs room up the path past mistwalker then waaaaay north along the hallway and back)
--- one thing to remember bout the basement, its the same as upstairs, big mobs no assist little mobs. Discordlings / girplan / puppies do not assist the gelidrans, and vice versa.
---- the mez is also a mem wipe, number of times its saved me while pulling shit from wierd ass places with a huge train following me, veeshan drops and you get mezzed, and the entire train disappears. Sometimes you can get a rezzer out if they get mezzed as well cause of the mem wipe.
You do not wanna ever go down there without a mezzer, cause you will at some point have an add if you are chain pulling.