Needless to say, they decided to scrap that plan and build the puzzle mechanics of Portal 2 with the same basic mechanic as the original and layer new mechanics on top.
At the same time, all markets can't be different, there is a need to have a common layer so that you can use the same services such as stock tickers and e-mail notification in every device.
It appears the major sticking point concerns the depth of the memory layer on which data is written: HD-DVD favors a data layer 0.6mm beneath the surface of the disk, the same depth as on the current generation of DVDs.