Criteria for Evaluating Web Tools and Apps

I'm often getting asked what my criteria are for choosing the tools, apps and resources that I feature on my blogs and in my teaching and do be honest I don't really think about it that much. I look at so many sites and apps that I just get a feeling when I see something that I think will work.
As that isn't a very satisfactory answer to give people I decided to start exploring my own criteria for what influences my choice. I'll also be running a workshop on this topis at IATEFL 2013 and looking at some apps and how we can evaluate them.

So far I have come up with a number of 'first impression' criteria that I think I apply before even thinking about 'how' I will use the app / tool.
Please feel free to browse the list and add any 'pros' and 'cons' or anything you think I've missed.
After my presentation I'll post links to some of the apps and tools we looked at during the session and what some of the audience thought of them.
I hope you find these criteria useful as a starting pont for your own reflections on why you choose to use certain apps and not others.
Best
Nik Peachey