I Googled but could not find a straightforward list of features.
What level can you go?
If it for during or after the MQ?
I did see there are Staves... How does this effect Mage play?

- crap load of armour, clothes and weapons
- new land space
- new outer plane
- new abilities, magic, shouts
- during or after MQ
- level cap doesn't change

I believe you need to be pronounced the Dragonborn by the Greybeards for the cultists to attack you in Skyrim, thereby initiating the DB Main Quest, but you can visit the island of Solstein immediately after downloading the expansion