What is the recommended thinset when tiling over concrete?
I use this type of Flexbond for concrete applications - well most applications.  This will isolate cracks up to 1/16th of an inch.  Also this stuff really bonds well to concrete.  I know that you can buy that $10 bag but I wouldn't skimp on the thinset.  I can say I have done a lot of tiling and not a crack yet (knock on concrete).

Also want to mention that I end up doing a lot of glass mosaic patterns (even in floors) and this stuff (get the white for glass) works great.