Note that DVD, AOD and HD-DVD are limited to two layers as a result of the way in which they are constructed. Blu-ray was designed from the start to support multi-layer and multi-level recording as the technology becomes available - so the current 50GB two-layer product is just the start.
Blu-ray is also based upon a similar data structure to DVD-RAM (and Minidisc), which augers well for its flexibility and potential user-friendliness in practice. As both Blu-ray and HD-DVD employ a 405nm blue-violet laser, they both need a second red laser to support CDs and DVDs - not surprisingly, even Sony's first Blu-ray recorder offers full backwards compatibility with CD and DVD (except for DVD+R/RW and DVD-RAM).
As Sony have recently released a DVD+-R/RW compatible DVD recorder, I'm sure that all the popular standards will be supported in second and third generation Blu-ray machines. :wink: