As someone remarked on Metafilter, the game is kind of like a slower Escape Velocity set not in space. This certainly goes a way to explain why I like it so much. EV was my absolute favorite game as a teenager, and arguably what got me into game development.
Let's compare SS to EV, then: the writing is much better and there is a whole system of skill tests and characteristics that the older game lacks. On the other hand, SS is really slow. I am in fact writing this post using the Steam overlay browser while my ship slowly goes from port to port in the background. EV is far more viscerally satisfying both in combat and in the simple experience of travelling between places.
As plenty of people have pointed out, Failbetter Games have a very grindy concept of gameplay, which means they think little of wasting player time with time-sinks like achingly slow movement across the map.
Of course, there's another comparison I need to make here: my own game, Space Exploration: Serpens Sector, a rather neglected project now that I have my hands full with Airships for the foreseeable future.
Much like in Sunless Sea, in SE:SS you move around a map between ports, exploring, reporting back home to exchange your discoveries for more fuel. You can upgrade your equipment and hire more crew. Your encounters are resolved through skill checks. It's frankly very similar. It differs in a few ways though: it's set in space, skills come entirely from your crew and equipment, and travel is instantaneous. There's no mode where you pilot your ship. You simply click on the star you want to go to, and the ship hyperjumps there, and the encounters for that star begin.
With SE:SS, I used to really worry about two things: that people plain weren't interested in that kind of game anymore, and that the lack of direct ship piloting was a massive flaw. Sunless Sea has cured me of both worries. It's a very successful game that's very similar to SE:SS, and its major weakness is exactly the presence of a boring piloting mode. Of course, I could put an exciting one into SE:SS, but that would really shift it away from RPG/exploration game to 2D shooter with RPG elements, which is not what I want from it.
I still can't exactly commit to when I'll resume work on SE:SS. There's a perfectly nice prototype, which can be improved by the sparing application of lessons learned from Sunless Sea. But a prototype is one thing. The game needs graphics, more content, polish, testing, marketing and so on. And there's other prototypes, equally good, and perhaps more fitting as the next game after Airships...