your old world and character are fine unless you crawled all over the mistland biome before it was out. I mean peeking is OK, but every area you looked at won't generate the new stuff is all. Its unlikely you broke the game this way.

you can also cheat in a set of pre-mistlands gear (people can tell you what is appropriate) and just start from there.

as far as it goes..
- a full area (new enemy, new gimmick for the area)
- a new boss
- new crafting materials and stuff to do at home (not to ruin too much but upgrades to crafting stations, new crafting stations, new gear to make)
- a totally new way to play the game
- a new dungeon to crawl through

and if you have been gone a WHILE, there were dungeons added to the mountain biome and a new crafting resource and enemy and such to the plains and other goodies.

That said I played it pretty hard and have only just now finished the mistlands in terms of beating the boss and having all there is to have (minus 1 rare cosmetic item). So there is a solid month of content here assuming 1-3 hours a day and that you have other crap to do besides explore the new area (like finding the new area, sailing all over the world for that, continuing to process food and keep the fires burning, whatnot) that eats up some of the time.

A lot of gametime is expected to be in enjoying building homes. If you just rush to the boss and call it quits, you are missing a lot. I didn't rush the boss, but all in all my mistlands adventures were more efficient than usual and probably than the average player. I got lucky finding a good instance early on.