Interesting convo... I'm usually one for throwing a few cents in that weren't thought of before (else, why comment), so in addition to the ones already mentioned that I like:
- DVBH
- solar
- attach to car/bike with specific accessory cradle-like item that changes literal mode and UI of Jolla

I'd like to throw this out there
- the other half allows Jolla an abiltiy to separate the side carriers program from the side users/developers play with. But the addition of carrier-specific covers, functionality can be enhanced or reduced. In something of a switch, a carrier defined cover could program the allowance of other SIMs in the Jolla, but not an outright changing of functonality
- sell the tech for back covers only to regions like Japan and SK where seasonal releases of mobiles also have a color/font theme and meet with fashion. Make these exclusive so that other markets would be inticed to do the same for other makes with covers/cases, but none with the same kind of attention to personalization as Jolla can. Can totally see DoCoMo doing this.
- position this alongside QNX as an automtive play, but only for verticals where the addition of mobile into logistics makes sense. Mobile is owned by driver, plugs into truck for trips, comes out and gets plugged into case to have package signed, plug back into truck. Discern analytics, and company can not only guage travel, but has a data bank they can use to improve efficiencies in other areas.

Another thing
- imagine all of those "offices of the future" concepts where folks are putting their mobiles into a dock and then a larger screen and keyboard (UX) are made available (think: Ubuntu Mobile). With "the other half" here we have an abiltiy for a hardware maker (Ikea, Wallmart, etc) to place the pripietory tech into that faciltiy and then turn the Jolla phone into a Jolla environment.

In a very real sense, and not the faster carriages kind that has made a lot of noise in this discussion, Jolla's other half tech is teh ecosystem. And its ability to play off the fact that accessories are where people personalize their device should be something that not only makes Jolla attractive to carriers where their's a rising middle class, but also be a slap in the face to Android which has had the potential for similar for years, but has left it to being nothing more than skins and homescreens.

When Jolla is ready, they will explain the tech. But, they could also pull an Apple and never explain it, making this tech one of many ways they keep themselves funded and attractive to companies that would like a well-done hardware outfit to replace a non-performing one (Microsoft, HTC, Google all come to mind).