From the sound of things, it's story mode only (no explorable), won't have a unique token, and only exists to wrap up the Flame and Frost story (The dungeon is a Molten Alliance weapon facility). It's by the same folks who did the Halloween and Wintersday instances, so not a full fledged dungeon.
This is sort of like their one-time Karka event, but done right. Everyone gets to see it, and (probably) get a unique reward for it (article says "really cool ones" that "you'll want to come back for."), but it's not over in an afternoon so there's no rush. But when it's gone, it's gone; people can look back at screenshots or unique items and say "Oh yeah, back in the day, you should have been here for that, it was so cool." In the same way people will eventually see the Queen Karka in Southsun and wonder what the hell that's all about.
It depends on how long 'a while back' is, but they've been getting plenty of updates with each patch. Also, as far as PVP/WvW/Endgame is concerned, GAF isn't the usual elitist mega-guild. Play what you want, nobody will ever say "We're only taking profession X" or "Kick him, he's playing profession Y". In the rare instance we've actually seen that, we are quick to curb it.
So if that's a concern, don't worry. Play what you like. People on official forums are usually assholes anyways.