The full list of features is in the link. But I'll sum up the major ones.
X-tended graphically improves the game. Much better skyboxes (or whatever they are considered in a space game...backdrops...whatever...), and much better planets. 2.0 gives each race their own stations. Instead of all of them being cloned. Among other graphical advancements.
Gameplay wise. There are a lot of dynamic events that happen. The main ones are pirates invading (and sometimes taking over) sectors or other races taking over their enemies sectors and giant battles. Sectors are bigger, and over time more of the galaxy opens up (so it slowly expands, so at first not every sector can go in). They changed the combat as well, improved it quite a bit...but to be honest, I don't fully recall vanilla X3 combat...so I'm going off the features for this one.
Lot better exploration for those that like that. As you explore new sectors, you get rewarded for it. More abandoned ships to take over that you can find while exploring. I haven't found any yet though.
Lots of new ships, missions and modules and stuff. A lot more content overall.
2.0 is quite an improvement over their previous 1.2, but its still pretty similar outside of improvements to the dynamic aspect of the universe and graphics, and some other new features.
If one has never played X-tended, it will be pretty much a new game experience. Since it is very different than vanilla X3.