I have a problem with mobile-first design.
I spend a lot of time every day sitting in front of three 1920x1080 screens. That's 6,220,800 pixels to play with, and web developers are not using them well. Take Twitter.com, for example: the Tweet column is 496 pixels wide. That's 26% of the width of just one of my screens for all of the content on the site that I'm supposed to read and engage with. When I'm sitting 3 feet away, the text is small, and it's a small target for my mouse (I've sped up the cursor so I can efficiently pan across screens).
When you build a website with a fixed-width layout, or when you build a mobile-first application designed for very small screens, you are leaving me and others like me out of the equation. I am a power user, and my ergonomic keyboard and I are much more likely to share and engage with your product than someone on a tiny mobile phone with a crappy internet connection and no keyboard. Your website design is telling me I don't matter.
I know I can zoom in. But I shouldn't have to.