I might not be the best to answer, but I do not believe any features have been removed in Express. I haven't downloaded yet, as I would rather do so when I can give the full program (PRO) an extensive workout. I don't think the feature list is accurate. In other words, the noise reduction sliders have not been removed based on the tutorial that was posted (but has since been removed - so perhaps something wasn't correct about the tutorial). I am also quite sure nothing has changed regarding how the histogram works.
Removing a pretty essential feature (noise reduction) and then trying to charge nearly two-thirds of full price for an upgrade less than two years after the introduction of the previous version would be not at all smart marketing. When v5 Limited came out about three years after the previous version, users complained that there was very little meaningful new so Phase One actually added the Basic Color Editor and Clarity features in v5.1 to attract more adopters (I was one of those who waited).
Interface and workflow in v6 is almost the same as v5, except you have some new features such as vignetting, slideshow, and a variable-setting loupe that works not only on the selected image but on the filmstrip (very nice). Presumably, there is some kind of new video integration (but I don't do video from a still camera, so I don't know much about that). It also looks like you can edit EXIF. My guess is the basic processing engine has not changed (which is very good, despite the somewhat marginal noise reduction quality). I do think the loupe will potentially improve the precision of the noise reduction, though. Finally, some engineering changes move things onto the video card so image adjustments will be rendered quicker (I thought v5 was already pretty good in that regard).
Bottom line, if you did a full evaluation fairly recently, you won't have any confusion while checking out the new version. As usual, a very intuitive workflow set up.