One of the most interesting features touted by the 360 and PS3 have been the integrated next-gen disc players, including optional HD DVD support for the Xbox and a Blu-ray drive being standard on the PS3. Once the PS3 starts to wane, though, and Sony looks to future consoles, it seems they are looking to do away with with optical drives. This could mean they are looking to potentially make all content online-only or downloadable-only, something we are already see some companies experiment with. Currently, we see things such as the EA Downloader and Steam. While both have been the subject of much criticism, both seem to have had a lot of success in content distribution.
While the PS4 is years and years off, it's still an interesting notion and one that poses a lot of more questions.