why is it different than other sites like Reddit or 4Chan.
The userbase. It's a smaller group of people that are active, and it's small enough that you actually recognize the people that post and kind of get to know their views and enjoy hearing responses from certain people. Also, unlike 4chan and Reddit, the site hasn't devolved into memes, reposts, and images with text on them that dominate the top "content" of what seems like every subreddit. People aren't posting things to get imaginary internet points or reach "the front page", and I haven't really seen any "whoring" of that type of submitting here yet at all. The upvoting and "points" of Reddit have turned it into the biggest circlejerk echo-chamber of opinion the internet has seen yet.
I stopped reading Reddit and came here because I like the smaller community, and I like the content here better. Sure, the same content could be found on the other sites you mentioned, but sifting through the pile of crap images and video game references that make up 95% of the "content" on those other sites just wasn't for me anymore, even the niche subreddits started to get crummy. There's far less content here than on Reddit overall, but I read and enjoy more of the things that get posted here.
Also on Hubski, if people post crap that I think is stupid, I can just ignore them, or the tag, or both. On Reddit, if a subreddit turned to crap, sure I could unsubscribe, but then I miss out on any content about that subject entirely, whereas here I can just unfollow/ignore those specific people ruining my experience and still see stories on those subjects. Oddly enough, the only people I've had to ignore here so far are the people who are/were prolific Redditors with gigantic karma scores... and guess what... my content and Hubski quality got better.
At the risk of sounding like an internet hipster, Reddit and 4chan both got too popular, and with that comes a massive decrease in overall quality. You know why Youtube comments suck? Because everyone uses it. You know why I think Reddit and 4chan suck now? Because everyone uses it. Hubski is still small and still has a "community feeling" to it which I like. But if Hubski had a couple million users by next week, no offense to MK, this site would probably get a lot worse too. It's the gift and curse of popularity that kills internet communities for me.
My two cents. You should use Hubski if you like it, it's really that simple. You don't have to use one or the other, and can use all three if you like. It's not a street gang or a professional sports team you're joining. ;)