Haven't played since launch. What should I expect? What changed?

[randomnamegenerator16]

As in title, I didn't play the game at all since launch. Leveled my character to 80 and stopped playing just as my character was about to kick ass of that giant ass zombie-dragon Zhaitan (or whatever he was called) in the dungeon. So, what exactly changed in the game since then? What can I expect? What will I lose by not buying expansion?

[Shakadam]

Summing up 3½ years of patches and content updates isn't gonna be easy :P

I would recommend Bogotter's series of video guides for returning players, there's 8 of them in total with the newest covering changes up until just before the expansion. You do have to watch them all though if you wanna know the changes that have taken place throughout the years.
Here's a link to the first one in the series: https://www.youtube.com/watch?v=Ce_d..._76_1w&index=1

In short, there's been many many game systems and UI changes since launch. Apart from the expansion there have been 2 complete story arcs (called Living World season 1 and season 2). Season 1 happened during a specific time and is thus no longer available to play, there's an NPC in Lion's Arch which will show you a recap of the season if you talk to her. Season 2 happened in more clear episodes and is still available (you have to buy the episodes from the gem store if you weren't online when they released).
Season 2 of the living world is a prequel to the story in the expansion and I'd suggest playing through that season before going into the expansion.
The main things you'll miss out on from not having the expansion is ofc the new areas, raids, guild halls (I think?) as well as gliding in both new and old areas and the mastery system.