Nokia Drive is one of the best offline turn-by-turn gps navigation systems out there, and it comes for free. The Jolla Maps app is based on the same framework (HERE Maps from Nokia), so I'd really like it to be fully usable in offline mode (ability to download maps for specific countries to device and have turn-by-turn voice navigation). This is essential when traveling abroad or in low-signal areas where mobile data connection is not available/too expensive, and I hope it is already on your roadmap.
Edit 11.5.2014: Answer "We're thinking about it :)" has been here for a while (since the Christmas). A lot of votes both to the question, comments and the answer. Any updates, Jolla? Worth a roadmap tag?
Edit 06.10.2014: There are another 207 votes on this related wiki post
Edit 19.5.2014: Cloud-based HERE services like synchronisation (via Nokia or even Jolla Account?) of favourite places and routes should also be implemented.
Edit 7.7.2015: Quotation from answer suggested s correct, the time gap mentioned is 6 months ago: "We aspire to offer this feature in the shortest time possible, however I don't see any possibility at the moment that this would materialize in a native form at least until next January."- Removed correct answer tag for this, looking forward for an update to the answer