Just installed 8c. I get no video previews when I play a clip from the project media window, just audio. I have to drag a clip to the timeline to get a normal preview. Worked fine with 8b (haven't changed/touched any preview settings).
Also - if I open one project in 8c, and then try to open another project, the video thumbnails in the project media window don't update - that is, the old thumbs from the previous project remain, even though the new clips are there (I can tell b/c I can preview the audio, though as I mention above, no video previews.) The thumbs of clips that have the same file names in both projects don't update - e.g., clip 001.m2t - the old thumb remains (they are different files in different folders so though they share the same file name, they are different clips).