Here are just a few. (Disclaimer: I use and love LastPass, but am familiar with 1Password).

Sync- it's possible with Keychain Access + Dropbox, but not seamless

Easy password generation- no switching to Keychain Access, copying, etc.

iOS access- up until iCloud sync, passwords were not easily transferred to devices

Support for Windows browsers

Form filling- Keychain will do addresses, but not credit cards

Windows and Android functionality. I use a Windows computer for some limited amount of things, and it's invaluable for me. Also, if you use Dropbox syncing, the Dropbox folder has some javascript magic that makes it work as a web app (decryption happens client-side).

On the other hand, the new keychain access will have direct Mobile Safari integration.

You can sync your passwords across multiple devices using DropBox. It will work with IE, FireFox, Chrome and Safari.

It will also allow you to share passwords (via Dropbox) between Macs and Windows machines. It can generate strong passwords as well.

Besides the items listed by others, 1Password also will store things that aren't web logins. Like SSH logins, or encryption recovery keys.