The biggest UI flaw for me is the picture view on the mobile Twitter-page.
They decided to replace the normal .jpg-link with a custom page, but this custom page is auto-fitted to the screen-width, meaning that you cannot actually zoom in on the picture.
I'm not sure you can even do that on desktop Twitter: if you can, I've never found a way to do so. I always end up right-clicking 'Open in new tab' instead.
Yes, this is very annoying indeed. Especially if people post comics or images with lots of detail: there's no way to get the full picture. The only way i know is right clicking and selecting 'open image in new tab', which is a hack.
They decided to replace the normal .jpg-link with a custom page, but this custom page is auto-fitted to the screen-width, meaning that you cannot actually zoom in on the picture.