For me, X3 was a nice improvement of X2 and more stable. However my essential wishlist for X4 for it to work effectively with the included 3rd party VTS' and plugins from other sources. I purchased Melodyne Editor when I had X2. Since I've upgraded to X3e, I've experienced nothing but crashes from it and the inability to update to the latest verison. Suffice it to say no-one has been able to figure out why it's doing this.
Addictive Drums 1 worked really well and when I upgraded to AD2, the visual timeline consistently stops for up to 3secs before resuming where the project is...the project keeps playing fortunately...however again, XLN has not solved this.
I have other issues going on with various plug-ins, but I won't go into them. I guess that some kind of checking system that identifies exactly what plug-in, or what other variable is causing an issue and maybe advice/process for fixing it.
Also when I send out my crash reports and select 'keep me apprised of this issue' is totally useless. I've not had one email ever that's come back from this apprising me of this issue.
So apart from all the bells and whistles, stability and reliability along with a clear process for finding out exactly why something is not work, would be my dream Sonar.