Season 6 is actually a big patch to fix some early season of the show. Go read this changelog:

Star Glimmer is the new addition to the Mane Cast.
More new fresh characters.
Trixie is return this season (Season 1 or 3 version required).
More songs.
An new Alicorn added to Alicorn race.
More new facial expressions and character movement.
Expand some old locations in Equestria.

Season 2 installment required:
Fix Dragon race stereotype issues in Dragon Quest.
Improve Spike performances.
Replace the executionmoral.inf in MMDW, which is glitchy and cause some BSOD when you run season 2 version.
Improve Fluttershy’s performances.
Fix some comedic issues.

Some technical problem:
Due to the comedic patches and season 2 maintainance updates (outdated version), season 6 is sometimes crashed if you run season 4 & 5 versions, especiallyRainbowDash.exe.
Starlight Glimmer is glitchy and often disappear in the background (?)
Please don’t hestitate to contact us for more technical issues.
More season 6 updates on the ways