1. Redirecting to the mobile page after the desktop page loads, taking up to ten seconds before i can start reading the content
2. Using some clever swiping / panning mechanic that relies on javascript animation. Yeah, let's build the heaviest ui on the slowest platform, great thinking there.
3. Preventing panning side-to-side and then having content too wide for the page (got this today on arstechnica, cutting off the bars on a benchmark graphic)
4. Asking me to install an app via a popup. On a content-driven site this is never, ever, ok. No site visitor likes this, and it doesn't bring in revenue. It is indefensible.
5. Presenting less content on mobile and offering no clear path back to the full content. Dell does this, i couldn't find out hardware details of their laptops from my ipad. No details, no purchase.
Ofcourse, there are cases where a little attention to mobile goes a long way. My phone operator redesigned their flash-driven website with html5 ... where the primary navigation are dropdown menus triggered by hovering and half the content doesn't load on a mobile device.