I have to admit, I like the flat button also but I appreciate you mocking up different versions. I dont think that a button needs to be raised to make people think they can press it. Look at the iOS settings page as an example. These buttons are completely flat. http://cl.ly/image/2j2m3I2R2x1z
Yes, but those are also the ONLY clickable options (and only visual elements, consistently in a list) on the entire screen. This does not hold true (in most cases) for a website / webapp.
The settings page is very specific context and there are no similar elements that aren't actionable so it is easy to learn that these links are links, not just labels. The arrow on the right side is also a nice cue.
You should not generalize from this example that flat buttonlike constructs are as usable as normal buttons in other contexts.
Also while I personally don't like the flat design, I believe it can work with a caveat: you will need to be much more careful in the design as you offer less conventional cues. In this I believe a design that offers more affordances is less prone to small mistakes.
