One thing I have noticed is that since the new plugin pinning functionality has come in (which is great).  Space bar won't start playback again if you have a plugin gui open and you have to click play on the control bar

I'll chime in too. This has been a minor, but ongoing, inconvenience for me as well since X1. For me, it's just related to the use of dim solo. Everything works fine with just normal soloing. However, whenever I toggle dim solo, on or off, I have to hit SB twice to get it to play properly. The first time, something doesn't play, or skips some note. The second time, it all works as expected. I did file a bug report way back when it first started in X1 using just Cakewalk supplied instruments (my stuff is mostly midi based with few audio tracks, if any). That example was very small, so it doesn't seem to be based on project size - at least not for me when I made the example.
 
BTW, I don't want to sound like I'm bashing here. I love Sonar and have been a Cakewalk user since Pro Audio days! I only mention it because you folks are discussing it. If this is the biggest problem I have, I'll live with it!