A fresh, new, responsive design with Curve2:
SMF 2.0 introduced the Curve theme, which was an update that gave a more modern web look to SMF. 2.1 includes a revamp of our default theme called Curve2! Notification Alerts:
In prior versions of SMF, notifications were a distributed affair, with the two primary options being e-mails and PMs. 2.1 changes this with the introduction of Alerts, which can be enabled to display all your forum notifications, major or minor, in a single location without cluttering your e-mail or PM inbox. PM and Post drafts:
Ever closed a tab or window on a long post you've been writing for a long time? [..]Then Drafts is for you, it is fully integrated into 2.1, along with the ability to auto-save posts and PMs as you're writing them.
Likes and Mentions:
Likes has been added to SMF, replacing the existing Karma system with a more content oriented one. A bunch of other stuff!
Full SSL support throughout the forum. (NOTE: Requires appropriate certificate and configurations on the server.)
Password hashing has been improved from sha1 to bcrypt.
The Admin area has received a number of usability and organizational improvements. A few of the areas have been reorganized and relocated as well as some overall clean up
The Moderation Center also has several usability improvements, including enhanced reported topics, and moderator notes among other things.
Improvements to membergroup requests and approval.
Permissions have been expanded with new permissions for member profiles, accounts etc.
Built-in support for Gravatars.
Users can select their timezones instead of manually selecting the time offsets.
jQuery is now included by default with many functions now using it in core.
Support for SQLite has been dropped.
Internal improvements reducing memory consumption and improving overall speed.
Lots and lots of other internal and external improvements! Ref. Source 2