Sure, I can tell you.
1. Access to the private Neoboard for Premium Members Only.
2. Increased random events on the site.
3. Your own neomail.com email address.
4. Extra random events in your neomail.com webmail on occasion.
5. Beta testing new games and features on the site, so you have first access to a lot of new things.
6. A Space Faerie Scratchcard game in your mail every Friday.
7. The "I Love Neomail" avatar if you don't already have it.
8. 1000 np added to your account every month you are a premium member.
9. Access to the Neopets toolbar. (Though I don't use it, nor do I see why anyone would want that, LOL)
10. The ability to "stealth" mode on your user lookup so nobody knows what time you were *really* on the site last.
11. The ability to see when all of your neofriends last logged on at one glance.
12. Acess to Neopets Portal, which can be customized to include some useful information all on one page.
That's all I can come up with at the moment.
I'm premium and signed up for the full year so if you have any questions regarding the service or my opinions, please feel free to ask me anything!