Soloable contents
99.99% of it. Even quite a few of the dungeons, that are designed for 5 player parties. Although it is just more time efficient to do things in larger numbers. Ive spent most of my time in this game solo, everything from PvE, sPvP and WvW. Almost anything in the game is achievable solo, with few exceptions from various categories.
The majority open world and dynamic events are soloable and all of the personal story up to the final (which is a dungeon).