Things you couldn't do until you hit 80:
1) Ruins of Orr. Pretty cool land, everything is dynamic events and there are no hearts anymore. You can solo if you want but its painful, this is an entire map balanced around running in a group. You could technically do this at 79 or so.
2) Enjoy being in higher level gear than others in WvW. Yes WvW bumps your base character stats to 80 but not your equipment. The sPvP is the one where everyone is the same. Although technically at 75 you have comparable gear.
3) Work on legendary. You continue to level at 80. Each time you level you get a skill point, you can use that buy stuff for your legendary.
Things to do when you reach lvl 80:
3) Craft better gear
4) Map Unlock
5) Ruins of Orr
6) Any dungeon and explorable mode (3 options or so per dungeon so 24 "heroic mode" branches in total)
7) Level your guild
8) Pretty much everything you can or did do <80.
So in a nutshell, what you do at 80 is basically what you can do at lower levels but a little more refinement and variatey. The only thing you couldn't do really till you hit 80 is gain a new skill point every time you level... oh wait shit... hmm I mean earn a skill point towards using it for your legendary weapon.