The real magic lies in the routing. Quite simply, you can route anything to anything else, and you can split any and every track into up to 64 channels. Let me give you just two instances where I find this so useful. There are countless others.
I can send all of my instruments to a submix (with just a few mouse clicks, by the way) and then insert a compressor to lower the instrument submix volume automatically whenever the vocal cuts in, getting the threshold to respond to changes in volume on the vocal track to. OK, other DAWs can do that. But REAPER can do much more. It can use sidechaining to control other compressor parameters such as ratio, and attack and release times as well. And not only compression. You can apply sidechaining to any parameters on any VST plug-in. You can get the EQ settings on one track, for example, to respond to changes in volume on another track. Or the wet/dry mix of delay on a track to automatically change as the track's own volume fade. Or anything you want really.
Parallel FX processing.
You can process track FX in parallel, in series, or in any permutation of these that you like. Here's a simple example. I want to add three FX to a guitar track – say Distortion, EQ and Reverb, but in parallel, not serial. With most DAWs you would either need to make two copies of the track or use three busses. With REAPER I can do that quickly and easily, all in a single track. I can change my mind and experiment without ending up with a workspace that takes up two screens just to show all the tracks. If I want to try out the Reverb and EQ in series rather than in parallel, it takes about 15 seconds to change it. Then another 15 to change it back. Suddenly. I'm limited not by my digital workstation behaving like it was a 1950s tape deck, but only by my own imagination and creativity.