DLC access for the duration of your membership and 10% bonus character progression (xp, gold, crafting research, and inspiration).

Edit:
Source: http://www.elderscrollsonline.com/en-gb/news/post/2015/01/21/your-guide-to-eso-plus-premium-membership

Ugly.Duck wrote: »

I've heard this is a cool thing to have, now I only know about the bonus crowns. What else do you get, game-wise? Extra XP? How much?

From memory you get:
10% bonus to XP
10% bonus to gold (from drops/chests/quest rewards)
10% bonus to crafting inspiration
10% reduction in crafting research times
Access to any DLC available during the time period of your subscription (you lose this access if you cancel your subscription, but keep any items or anything gained from that DLC during your sub)
The crowns that you already know about.