Aside from PCMCIA/ExpressCard upgrades, which are useless nowadays thanks to USB2/3, the only upgrades available to laptops are memory + disk. RAM is cheap enough where companies can toss in 8gb (soon, 16gb) without a second thought, which is more than enough for probably 99% of the customer base for as long as the laptop is in service.
That leaves the disk drive as the one non-upgradable part. I imagine part of the reason for this is the (relative) bulkiness of the 2.5" SSD form factor. Are there any other standardized sizes that could work in a rMBP form factor? 1.8" maybe? The 11" MBA may have problems fitting it, but a swappable 1.8" SSD on the larger models would be quite nice.
