To me, iPad Air is a great "companion" device but definitely not my "main" device. It can execute 90 % of the stuff I do on my rmbp and it even outperforms it in some tasks. However, I would not replace my rmbp with my iPad Air because of these limitations, which are critical to me:
-Screen estate on the iPad Air is too small to work on detailed or complex documents.
-Lack of pointing device is a deterrent for productive tasks.
-The "silo" or "compartment" approach for apps, plus the lack of a common filesystem to access and "share" documents is an issue. Plus, multitasking is quite lacking on ios (for instance, switch between application windows easily, copy/paste content from one application to another, having two windows side by side to compare content, etc.).
-Many full fledged applications are not available on the iPad: MS Office, MS Project, etc.
-Lack of Java is an issue to me as I need sometimes to load Java applets, etc.
Regarding MS Project, I need to work on project plans and schedules on a regular basis. I've installed it on my Bootcamp partition, as there's currently no Mac version available. This is one "real life" example, but others could require to run some specific applications that are not available on ios.