One thing which I would add regarding the affordability aspect is that in my experience there's too much emphasis on the initial cost of a device versus the longer term running costs and total cost of ownership. Of course it doesn't help that there's only limited information on the TCO of small- and large-ICT4E projects (with the IDB's "One-to-One Laptop Programs in Latin America and the Caribbean" report from 2011 being a notable exception).
Related to that point - and especially the long-term running costs - are two others aspects which I'd call (1) "repairability" and (2) "updateability".
(1) "repairability": Regardless of how robust a device is a certain percentage will always break and need repairs (e.g. see Plan Ceibal's breakage rate reports). Such repairs, warehousing spare parts, etc. are a challenge on many levels, require infrastructure, logistics, financial resources, etc. Certain devices (e.g. OLPC's XO laptops) and I'd argue even certain device categories (e.g. netbooks vs. tablets) have different design characteristics which make them more - or less - repairable which influences the available options for dealing with breakages.
(2) "updateability": In some ways this is quite similar to (1) in that in order to continue to be useful devices will need software updates. e.g. the 1st generation iPad was initially released in April 2010 yet the latest iOS version which it officially supports is iOS 5.1.1 which was released in May of 2012 (all data coming from Wikipedia). Similarly for Android Google talks of a "18-month update window when Google and others traditionally update devices". Such timeframes might be (barely!) acceptable when talking about regularly replaced consumer devices but for ICT4E initiatives which have a longer time horizont this can be a challenge. As app ecosystems tend to aim for the cutting edge of OS support owners of older devices are often left behind. This is an area where I feel that PCs and laptops degrade much more gracefully then more closed platforms such as mobile phones and tablets.
Anyway, just some additional food for thought... :-)