Easy to use interface, plug-ins, ability to export to many data formats, quick render....

Reaper is just a lightweight fast working software, you just launch it and it works fast, just adding tracks and start making music, the UI can be a little complicated to first timers but it's easy to get used to it, supports most types of plugins which are easy to add, and it can add plugins and effects to either the input of the track or the track itself which i find super handy sometimes while recording and live monitoring, using search for finding any plugin or instrument is a super handy feature, working with midi is different that other DAWs but it's not complicated, and the functions are great as well, the software handles the audio driver very good and very few issues occurred due to audio interface problems, the software is very reliable and rarely crashes, i consider reaper to be on of the most stable DAWs out there, you can rely on it to record really long multi-track sessions with no problems, i love the integration they have done with ReaRoute ASIO, it made going live with software like OBS possible, which i found this is to be very hard to do on Windows because of the way how Windows handles audio devices

The ability to create custom actions to do things faster.