A huge problem I see with responsive/adaptive design today is that, all too often, it treats “small viewport” and “mobile” as being synonymous, when the two concepts are orthogonal. A mobile device can have a high-resolution display, just as a desktop user can have a small display, or just a small browser window.
Responsive designs need to design for viewport size, and nothing more. It’s not mobile, it’s a small display. Repeat that to yourself about a thousand times.