So -do I have to be lvl 80 to play the expansions?
Yes you do.
But if you buy them you get 2 level 80 boosters which will instantly level up a character, and gliding, mounts and other masteries are account wide so once you unlock them on one character they're available to all your characters, even ones below level 80. It's not recommended for new players to use the boost right away because jumping straight to level 80 and especially going straight into the expansion maps is likely to be overwhelming, but if you've been playing for a while and know the basics that can be an option for quickly unlocking things you want access to (especially gliding and the raptor mount because those are unlocked right at the start of each expansions story).
Danielle Aurorel - Desolation EU. Mini Collector.
"If someone really cared, well they'd take the time to spare, a moment to try and understand another one's despair"
yes (how to start the expansion)
You can play the expansion even if you didn't complete the personal story, but you must be lv80 (to unlock the masteries).