I'll address the binary compatibility concern. I think at this point for the average scala developer, binary compatibility isn't really a big deal anymore. At this point the releases require very few (if any) changes to the codebases and all the library maintainers have stepped up their game with releases corresponding with new versions.

Sure, it'd be great to have binary compatibility, but I think the ability to make changes to the language unhindered by supporting byte code from previous languages is more valuable.

For example with the 2.11 release, I had three different non-trivial sized projects and for each I was able to simply change the scala version in my sbt file and everything worked just as it had before with no library or compile issues.

I will agree that it was a huge issue in the past with 2.7 - 2.9, but in my world it hasn't bitten me in a long time. Definitely not trying to say it isn't an issue or something that wouldn't be great to be fixed, but at this point I have a tough time seeing it being considered a barrier to entry in any way.

Also, I just throw in that for me compile time hasn't been an issue for me lately either. I work on reasonably sized projects, but definitely nothing on the twitter / foursquare scale.