Things I miss from Sonar in Studio One:
1. Step Sequencer
2. ProChannel (when it works correctly). This is somewhat alleviated with Brainworx's Console line of plugins, which contain well-emulated vintage EQ, Compression, and console channel emulation.
3. Drag quantize in PRV
4. Sonar-specific plugins (e.g. TH3 Sonar). Quick note here: Focusrite users can get all the AAS session plugins for free right now, so Focusrite users accustomed to these in Sonar can now port them to Studio One.
Overall, StudioOne is a bit of a paradigm shift, but a positive one. I found myself more productive on average in S1 than in Sonar, but I love the vast array of Content that Sonar comes with. Fortunately, most of it (loops, Dimension Pro, Rapture Session, VX1) can be used in outside DAWs.