I've been having stuttering issues with playback on my MBP17. I thought it was just the hi level of HD. Until I copied a video onto a thumbdrive and played it on my PC laptop. This was an H.264 video that played without one skip on the PC encoded at 20mbps. The same video encoded at 6mbps skipped a lot on the MBP.
Why might the Mac not playback HD video nearly as well as my PC??

Intel Macs with a 7200rpm drive have enough power to playback a single stream of 1920x1080 H.264 quite well, especially with Quicktime designed with H.264 playback in mind.

Tell us your Mac's specs and the software you are using, that would be helpful in pinpointing the problem.