A little late to the "party" here (day job, rehearsals, etc...), but here are my quick thoughts on a top 5, or at least the ones I can remember at the moment. :)
1) Staff View enhancements. This has been my long-time biggest pet peeve with SONAR, and one of the key things that has made me consider switching horses over time. I'm not looking for print quality notation -- I have Finale for that. Rather, I'm looking for a workflow-friendly notation view for use in the arranging process. For context, most of my tracks -- really, everything other than vocals, are MIDI -- softsynths, to be specific. In a typical pop, rock, country, etc. arrangement, I play everything live and arrange while recording. I may use Staff view for some tweaks when I'm trying to suss out something weird going on (i.e. conflicts of some sort), but I mostly just play the parts. At times, however, I'll add, say, a string quartet or brass section, or maybe something even a bit more involved. In such cases, I will usually want to write parts in notation -- I'll play them later to get more feeling, but I need to visually see what I'm doing with multiple parts. The horizontal scrolling of multiple staffs that we already have is needed. What is needed, however, is much more friendly note entry, including changing note values, positioning notes intuitively (i.e. not having, say, eighth notes come out on 32nd note boundaries when my mouse is where I think it should be to be on an eighth note or quarter note boundary), probably being able to input rests as placeholders (and helps on the positioning front), ability to input triplets in a friendly manner, etc.
2) Better behavior (no audible artifacts) in playing certain software instruments, especially Arturia VST3 instruments and the UJam Virtual Guitarist products, but also AAS instruments and others. These seem to get a lot of audio artifacts when playing them in SONAR prior to freezing (they freeze/render just fine -- it is just a matter of playback). This isn't a scenario of an underpowered system, and I don't have the same problem in Cubase 9.5 or Studio One 3.5.
3) "Cleaner" screen management when using a single screen (which I do all the time). What I'm talking about here is the ability to quickly/easily switch what I'm viewing as suits the task without having to define screen sets, bring up dialog boxes (i.e. the Track Manager), etc. This is not only about tracks, but also buses. I find myself frequently having to maximize one thing or the other to get enough of what i need to see on the screen, and the bus pane is usually kept low (or hidden) to give room for more tracks, but then it's hard to get a bus large enough "in the moment" when wanting to make a quick adjustment, and there is a lot of juggling of bus visibility. Cubase has something similar to the SONAR track manager in their equivalent of the Inspector, where it can be always visible, or switched to as a tab (like we switch between Pro Channel, Track properties, and Clip properties). No dialog boxes required. I've found this quite efficient, especially with tracks and buses keeping in the same areas -- i.e. no separate track pane and bus pane.
4) Better VST/VSTi performance. It does seem like I can get more going at lower latency settings in Cubase 9.5 than in SONAR before getting any artifacts in the sound. I think the same may be true of Studio One 3.5, but I haven't used that quite as much (haven't actually finished a song on that one yet). (And, just for whatever it's worth, Studio One really doesn't meet my general needs very well at this time, so I'm not likely to make that my main DAW. I'm not sure about Cubase at this point. My long history with SONAR would make me prefer SONAR's successor, all other things being equal.)
5) A solution for a relatively new SONAR issue I've been experiencing where starting up a project that changes sample rate from what has been set on my audio interface (MOTU 828x if that is relevant) seems to get into a tough-to-interrupt loop where you get the brief burst of white noise that usually happens when SONAR (or another DAW) changes the sample rate then gets an hour glass for a long time, eventually has a really tiny break where queued up mouse movements/keystrokes happen, then repeats the cycle. This didn't use to happen in SONAR. It used to change the sample rate to match the project cleanly, only sometimes needing a bus to be reassigned (and the brief white noise burst), but no endless loop thing. Maybe this came with a Windows 10 update??? I think it is newer than the most recent SONAR update. It is *very* frustrating, though, and I'm not seeing it in Cubase or Samplitude. (Studio One has other issues with loading a project at a different sample rate.)
I should probably mention that, when the Gibson announcement about Cakewalk happened last year, I decided to take advantage of some of the crossgrade deals to educate myself on some of the other DAWs (I eliminated Digital Performer, but picked up Cubase, Studio One, and Samplitude). I've mostly been a happy SONAR user, aside from the long-term Staff View issues and more recent issues with VST3 plugins. There are, especially, many things I like about SONAR's UI (the ability to categorize plugins as flexibly as we can do now is particularly useful). But I also decided to try alternating between DAWs when doing different projects, both to educate myself more on what is out there, and to consider what my primary DAW should be in the longer term. (My blog on this is at
http://rickpaulmusic.com/thoughts-on-the-demise-of-cakewalk/.) I am, of course, hoping SONAR's successor under BandLab will come out on top, but I am keeping an open mind on this at the moment.