Jumping in with my experience working w/ Valve re. release of Dredmor and expansions via Steam. Note: This is largely speculation based on my experience and some flimsy circumstantial evidence, not actual inside knowledge, and not the opinion of Alex, Fractal Softworks, or Gaslamp Games, etc.
A word on Steam:
It is perhaps the only way to meaningfully distribute a game for the PC. It blows every other distribution platform (Desura, Gamefly, etc) out of the water in terms of sales volume. By a large margin. No, really - it's dramatic how much of the market is concentrated on Steam. Considering how well Alex has done selling Starsector completely independently, he's going to do *very well* selling it through Steam.
(Actually, the Apple store might have decent volume, but it's kind of a different market and I don't actually have experience with them.)
Getting on Steam & thoughts on Greenlight:
Gaslamp Games was lucky to get in before the Greenlight mess. It would *suck* to have to play that somewhat broken and chaotic popularity contest especially as completely out-of-money indie devs (which Gaslamp was when we finished Dredmor). Happily Alex has income from Starsector already, so he wouldn't be screwed by that.
As for Greenlight, it's really strange and I'm glad we didn't have to put up with it -- Valve has a unique approach to running a business (check out their employee handbook if you haven't read it already) and it shows in their practice. They are essentially business via science, by running experiments, adjusting based on results, then going with what works. This means their behaviour can look erratic from the outside, but there really is a rationale to everything they do. The other side of things is that, as you can see via the employee handbook, any initiative or project needs an employee to advocate for it by choosing to work on it. A lot of the un-cool stuff can get sidelined -- a good deal of quality control of distributed games (see: The War Z or whatever it's called, and all the crap that was on Greenlight before they had any filtering at all) is not done until there is a backlash that requires a response.
Pre-Greenlight, you had to get the attention of someone in Valve to get on Steam -- get an advocate, ya'know. They had *one* contact for new product inquiries who I'm sure got bombarded like crazy by indie games - and tons of crap shovelware too. There was a lot of outcry over why this or that game didn't get onto Steam, but seriously one person could not be expected to go through all of the applicants and judge whether they were any good (hence Greenlight as an effort to externalize this work onto the public). Gaslamp was lucky because we hired on a Serious Business Guy via a family connection who worked in the games field who knew how to properly approach Valve so we hired him on contract and it seems that they wanted what we were making.
Back to Starsector:
Alex might be able to pull through on Greenlight because he has some good PR via TotalBiscuit, a number of game blogs, & others. Heck, I know PC Gamer would like to cover Starsector once it's in a more complete state, and Rockpapershotgun has featured him before and surely will again. Although Alex doesn't play the hype, preview, and publicity game at all (much to some people's annoyance, I know), he simply makes a damn good product and that quality speaks for itself. Valve people follow all of these sources and if he gets the praise the quality of Starsector will surely get, he'll be good to go.
(Plus, he's got me & Gaslamp behind him if he needs to use our connections.)
In short, he'll be fine.