After Transpo1 mentioned Shane Hurlbut (thank you!), I went on the B&H Audiovideo website and watched the excellent series of DSLR instructional videos that Mr. Hurlbut has there on using DSLRs for shooting cinema. I have never used a DSLR for video, but from watching the videos and reading other blogs and reviews on the subject, I have gleaned the following needs for a large sensor HD video camera that are not adequately addressed on the current crop of DSLRs, if they are to be considered more versatile in HD production.
1) The ability to shoot continuously if needed (i.e., for events, lectures, etc). The present Canon DSLRs are limited to 8-12 minutes of continuous shooting, and, I believe, do not have multiple cards or hot swapping ability. Mr. Hurlbut also mentions IQ degrading as the camera heats up (more noise).
2) Output through HDMI is often limited to SD or 8 bit and does not allow off-camera capture of HD to the higher quality that the sensor is capable of. HDMI is also not as secure a connector as HDSDI, so the likelihood of losing your cable feed during shooting to a remote capture device or external monitor (or both) is an issue.
3) Shane Hurlbut mentions in the DSLR videos that he captures sound separately from the 5D MkII, so that gets around the lack of professional balanced audio inputs like stereo XLR jacks that dedicated video cameras typically have. In a studio or location setting with relatively short takes and a bigger crew, separate sound is more practical, and the sync issues can be resolved with clap boards or other tried and true methods. For a more general camera with only one shooter that becomes impractical. I do not know if the DSLR audio circuitry itself is up to the better sound with, for instance, a Beachtek-type adapter that has dual XLR jacks, or not.
4) Finally, focus pulling is also much easier with a larger crew and cinema-style lenses that have repeatable distance readings on the side of the lens, where the focus puller usually is situated. Otherwise, power zooms with good focus assist or decent autofocus are helpful sometimes. I have heard from professional editors that a lot of DSLR HD video footage comes in badly out of focus ( a problem that HD accentuates, anyway).
Thank you, contributors, for your great comments!