Amongst the new features which will come in handy (at least for me) are:
- Support for Java EE 6
- Support for Java SE 7
- An improved Griffon/Grails plugin
- Android development-support
Anyway, as I said: the most exciting bit for me is the fact that IDEA will be open source which means that a greater number of people can get accustomed with IDEA; yet, I speculate that the real benefit lies in in the fact that you can now step into IDEA's code when the plug-in you may be writing just doesn't seem to do what you expect.
To round this off I should add that it's not all "Netbeansy". Despite the fact that the guys behind IDEA chose to go open source, they didn't go all the way. If you, like me, use the Java EE- and Spring-capabilities, you will still have to shell out roughly €200 for a personal license of the IDE. All in all I am still pleasantly surprised and I am downloading the source code as we speak (/write).