I would say just one thing: consistency, consistency, consistency. Yes, I repeated it, because I think it's important, but also because I think there are 3 string to it (at least)... So wat do I mean by "consistency"?
- Consistency of user experience - ie. use the platform's standards (eg. ctrl-C=copy) and don't re-invent the UI/UX wheel. SONAR does a pretty good job of this; other DAWs don't (in my experience, some of which is old, to be fair).
- Internal consistency - use the same look and feel, terminology and menu structures across your application. SONAR doesn't do brilliantly on this (in some cases, this is understandable as plug-ins, say, have been added or bought in at different times from different vendors, and plug-ins are probably less critical since there are always third party plug-ins that people will buy which will look and behave differently from packages ones), primarily due to "legacy" screens and menus, but not entirely - eg. the Ripple Editing options should behave like the meter selection options as they are both selectors, but they don't
- Industry consistency - use the same terminology and processes that already exist when adding a feature. Reaper is the worst at this (have you seen its naming conventions?!?!), but SONAR could be better - eg. when adding side-chaining functionality, most (all?) other DAWs are driven from the side-chaining plug-in (ie. open the plug-in and select the sc input from there); SONAR does the reverse (which I actually think is a more sensible approach, but it's not the industry standard way of doing this, which increases user confusion).