The price.
Plus I have mine setup to basically serve as a super-fast idea capturing device. I have the key presses Shift+?, Shift+>, and shift+< bound to the "create track," "arm selected track," and "start recording" commands, respectively, so the time between firing up the program and seeing an active track recording my guitar takes maybe 5 seconds.
Also, and I'm sure all decent DAWs do this, but it's really good at organizing your files if you take the time to set it up to do that. Every session I save automatically gets its own subfolder in my "Projects" folder, and in that session's folder is the main project file and any renders I make, then all the actual audio and wave form files are contained in the "data" subfolder in that session's folder. This means that every project I do is completely modular and can be moved freely by simply dragging the main folder around, and as long as I point Reaper to the project file, it does the rest.
So basically, I'm not sure how it compares with all the other software out there but for me it's great at lightning fast idea capturing and organization for when I want to quickly relocate and revisit stuff later without much fuss.