Good for you but understandable user interfaces are (and should be) higher priority than prettiness; the "warning button" looks like a warning message for me.
The problem with your example is context. OP's link to the flat design is lumping all the UI elements together. Essentially, out of context. Even your use of the label was awkward. "Danger Button", the text, is weird. However, if you'd relabeled it Submit, or some other common action word, it would make more sense.
Couple that with the lack of a form.
I'm not saying that flat buttons are superior, or that 3d looking buttons are worse. Simply pointing out that context matters, and even your opinion is worthless. That it looks like a warning message to you is indicative of other problems, not merely the lack of 3d.
The problem with your argument is the need of context; not everyone designing web pages is good at design and contextualization of interfaces, this is not an "opinion", is a FACT. And what it means is that in general, promoting practices such as flat buttons is counter-productive for usability.
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.
It only takes mousing over the button to get an indicator that it can be interacted with. That combined with placement should be sufficient to communicate purpose.
An over-reliance on a hover state leaves tablet users in the dust, though. I see no reason for completely flat buttons, as it's eminently possible to design ones with subtle depth that play well within the bounds of "distinctly digital". The principle that your users need to touch everything in order to learn their boundaries is a dangerous gamble, and may very well be leaving conversions on the table due to risk-aversion.
The new Blackberry OS is like this with gestures. Sure, you can spend 20 minutes moving your finger(s) in every way imaginable to learn how to use it because there's no chrome (or hardware buttons), but it certainly doesn't instill confidence on the first use. Instead, it presents itself as a challenge: "learn how to use me, I dare you."
> An over-reliance on a hover state leaves tablet users in the dust, though.
Not only that, it requires a mouse user also to hover over all possible elements to just know what's clickable. Kind of like some of the first graphical adventure games.
Or imagine a website where a link only turns blue and underlined as you mouseover it.
"Input" versus "Indicator" is a big problem in FlatUI that needs to be solved, there is no shared visual language yet to signify which is which, and it needs to be possible by just looking (and preferably other modalities as well)
