First I want to say that it has been a great experience and this is not a rant topic. I've learned a lot of useful things about making games and I've made a decent amount of $ .
While I like C2's approach of easy visual programming and clean interface, but there are 2 big reasons I choose to take a break and try another engine:
Reason 1: C2 uses developing/unfinished technology
Reason 2: Limited commercial possibility
Desktop: Yes, exports an .exe file and that is all, there is no official Steam plugin, the 3rd party one is a lot of work to implement and is hit or miss.
Websites: There is support Kongregate but should also be for Newgrounds and others.
Mobile devices: You only get the HTML files. Good luck with all the 3rd party warpers that are somewhat hit or miss and no constant fps. Also, very little support for ads vendors. And you can't use Physics due to CPU load.
I'm not against of 3rd party warpers, the problem is that Scirra has no control over them.
Also i dislike the lack of 3D (and the motivation not to implement it in C3). I know that C2 is for 2D games (3D models are harder to manage then 2D sprites ) and I agree with that, but some simple 3D objects like the ones in Construct Classic would make great difference (example: New Super Mario Bros or Sonic Rush for Nintendo DS)
That being said, I would recommend C2 to anyone who want to start creating games and I'm sure that C3 will fix a lot of C2's current shortcomings, but if you plan to make a big project, you should think twice.
This doesn't mean that I give up on C2, it is still the easiest, fastest and optimized HTML5 game engine i know