We call responsive design a design for a website depending on the device used to see the website. With responsive design people won't see the same thing on a desktop computer and on a smartphone.
This looks like a good idea. If you see a full website page on a tiny screen you can't click with enough precision. With responsive design you see less elements on the page and it's easier to click on something.
Difficult to use
It's easier to click on something with responsive design. But it's not easier to navigate. When you are used to the interface of a website it's really annoying to search for the items you usually click. Menus are not the same, buttons are bigger, layout is changed. You've lost all your references. You are disoriented.
As a user I always try to quit the mobile versions of websites.
Hard to implement
As a programmer I find responsive design to be very difficult to code well. How will the site look like on a tiny screen in landscape mode? In portrait mode? You have to consider many possibilites on the client side. Now you add all the edge cases into a CSS file which looks messy. After hours of work it's done, ready to ship.
What happens when you want to change a big component on the interface? Probably you will have to do the CSS work again.
Who loves reponsive design?