Thanks, Michael, for another great post. For me usability is the most important aspect. As you have argued (and I very much agree with this point): "the best technology is often the one you already have, know how to use and can afford." I think that usability can be further developed into a sixth challenge ...
The trend is towards mobility: PC --> laptop --> tablet/mobile phone. As the learning device moves around with the learner it has the potential to, and in many cases does, become intertwined with his/her life activities. This could be using it to communicate with friends or family members, get the latest sport scores or play games. Parents may use the device for livelihood information, such as getting agricultural tips or market prices for their crops (to give a common example).
While there is a case to be made for devices with a focused feature set (reading on a Kindle offers an experience with less distractions than reading on an iPad, where apps and links are waiting to be clicked), for me the ideal device is one that does not only serve an educational function in the traditional sense, but can also serve some of the life needs of its users (learners) AND their families, friends, etc. In a world where devices are not yet in abundance and are, more often than not in developing countries, shared among family members, wide usage of limited resources is important.
The educational benefit of this approach is that students learn additional (life) skills (online communication, gaming, etc.) and their parents, who also find utility in the devices, increase their support for the technology. Of course there are challenges related to this multi-utility of devices, such as a parent or sibling hogging the tablet while the student needs it to do homework. But overall, as technology becomes increasingly part of our everyday lives (not just our "learning" lives), multi-utility will be the trend.
So, I might add one last challenge that builds on usability: life utility. (Feel free to come up with a more articulate label for it!)