ripple editing,
stretch markers (similar to sonar's audio snap),
track (+plugin) processing load balancing and performance monitoring (of individual tracks),
batch export (using for ex. region matrix),
actions and programming functions (for ex. you can define custom actions to be performed when reaching desired marker etc. - this one is really advanced stuff, one of reasons for ex. Ubisoft is using Reaper when developing audio for games),
grouping and folders unified (sonar implementation of folders and mix bus groups is sadly schizophrenic, for ex. you cannot solo bus and different track to listen simultaneously in sonar)
audio/midi/instrument/group/whatever tracks/items/clips unified (no matter what you put in the track, it works - you can even put midi and audio clip/item into one track, or multiple tracks into one track)
borderless customization options (not just visual layout, keyboard shortcuts, but literally everything)
portability (possible to install on usb stick and carry around)
settings migration (possibility to move entire Reaper settings across multiple PCs, everything is stored in files, not some silly Windows registry)
project files are plain text (unlike Sonar project files, even without having an access to actual daw software, it is possible to check or even edit Reaper project files)
multiplatform interoperation (Windows, Mac, Linux)
ps: I have always been leaning towards Sonar since version 8.3, found it superior compared to Cubase or Samplitude or ProTools,
it's a weird coincidence that Cakewalk's announcement of Mac version sparkled in me an idea to try build a Hackintosh and start messing with Mac overall, and after skipping Logic Pro (as it makes no sense to start learning some non-multiplatform software) my eyes were set on Reaper, which is not burdened by "legacy" code and workflows