The multiplayer aspects are very nascent/underdeveloped. That MMO aspect will be better developed in the future, but as of right now, the latest big patch kinda screwed up text chat for myself and multiple others. You may or may not see other players flying around, and depending on where you are, you may go days without seeing another person, as has been my experience. I've seen only about 5-6 people since gamma started, but I've also spent 90% of my time at the periphery of inhabited space.
That said, I really enjoy this game a bunch, spending most of my time trading and exploring, but improving the multiplayer is paramount to this game's longevity, I think. At minimum, adding a chat channel in stations would go a long way to creating an in game community (flame away forum trolls). One thing this game suffers from, is the fragmentation of the community. There are private groups people can join where they will ONLY EVER see other people from that group in game, along with npcs. There are groups with thousands of members, who will never see the light of open play. There are also the people who only ever want to play solo. So what's left is high degrees of fragmentation. Regardless, the game does not at all feel like an MMO/MMORPG, and rarely feels like MP, from my experience. But again, I still thoroughly enjoy the game for what it is.
As far as the economy, humans and npcs both, are supposed to have an impact on supply and demand at all the stations (I'm PRETTY sure the npcs have an effect, at least). No crafting, but there's plenty of ship modification you can do through purchasing module upgrades, with different types of stock available at different stations (and there's a great wealth of different types of upgrades you can buy for your ships).
You can currently do the following for professions:
Trade
Mine
Bounty Hunt
Smuggle
Pirate
Explore
Some or all of these may or may not include plenty of combat, depending. The biggest limitation is imagination, I think. I would look up game play videos on youtube or find a streamer on twitch.tv to get an idea of what to expect.