In Develop mode, I often adjust a slider by clicking its name with the mouse, and then using the + and - keys to adjust the value. (I must be the only masochist who does this, because apparently it's never tested by Adobe, and it's always buggy.)
In Lightroom 4.1 and earlier, when I selected a slider with the mouse, that slider stayed selected when I edited a different photo. For instance, if I clicked on Contrast, the + and - keys would adjust the Contrast as I moved from photo to photo, until I clicked on a different slider.
In Lightroom 4.2, regardless of which slider I click on, the Exposure slider is always selected when I switch to a different photo. So if I want to adjust the Contrast of each photo, now I have to click on Contrast for each photo, before using the + and - keys. When combined with the performance degradation of LR4, this makes Develop mode much slower to edit hundreds of photos.