So there's been comparisons of Reddit and Hubski, but relatively few of Tumblr and Hubski.
I'd like to take a moment to share my observations. I don't want this to become a Tumblr hate-fest, but I do want to be critical.
Tumblr's userbase is almost the antithesis of Reddit's. Where Reddit leans toward right-wing 20's-30's males interested in STEM fields, Tumblr users are more often than not teen-20's left-wing females focused on social issues and art. Hubski's userbase appears to hew towards the middle of all of these spectrums. Or rather, there is a certain level of diversity in which various extremes are not over-represented. (I'll admit I'm no moderate in some regards).
There is a certain level of group-think which has developed on Tumblr. Even when I have tended to agree with it, it still has proved a little frightening. The epitome of this for me was the "trans asterisk" episode. People of a certain age involved in LGBTQ communities have been accultured to using the term "trans*" in order to be inclusive of people whose gender identities fall into a gray territory not covered by the male/female dichotomy. This terminology has been used since at least the early '00's to make sure these people's existance and presence was acknowledged and welcomed. Then around late 2013 there was a swift reaction against the asterisk. The Tumblr consensus claimed that the asterisk actually represented the exact opposite of what those of us in our late-20's and 30's had taken it to mean all of these years. Now, there were a few in-depth, well-considered posts that inititated this change in the tide, but they were quickly overwhelmed. What I saw on my dashboard was a sudden roar of posts to the effect of, "If you write 'trans*' you are a bigot and I will immediately unfollow you." What should have been a cordial discussion about a nearly trivial point of syntax quickly mushroomed into a slew of judgemental, malinformed ultimatums. It took months to blow over. By the time I actually found the posts that made some decent arguments against the asterisk, I was already disabused of Tumblr users' pretensions to individuality and critical dialogue. (Runner-up: "Don't say 'stupid,' that's ableist.")
My experience of Hubski so far is that divergent viewpoints are generally welcomed, so long as they remain civil and non-trolling. Indeed, I've already had challenging and productive discussions with people I don't see eye-to-eye with. While that's not impossible on Tumblr, it's certainly not the norm.
Group-think also exists on Reddit, of couse. But it tends to be a product of, and confined to certain subreddits. Which is not to say that talk of transhumanism or "alphas" and "betas" doesn't leak into evidently unrelated topics. When they do they might be carefully considered, dismissed or accepted. The reaction depends largely on the culture of the particular subreddit. The ideas that are widely accepted on a large part of Reddit still don't have the site-wide hegemony of some ideas on Tumblr.
tags: There's some differences between the ways both sites use them. I find it interesting that Hubski only allows OP to use two. This artificial scarcity seems to necessitate the user to think carefully about which to use. Tumblr users often use very customized tags. These can come in very helpful for looking for a very specifc kind of content. The most infamous tags used on Tumblr are TW (trigger warning). I never understood purpose of putting "#tw gore" instead of just "#gore". This is where the tag functionality on Hubski beats Tumblr's. Hubski has a built-in method for blocking tags. I noticed instant improvement in my feed as soon as I blocked #sillyseason , for example. Whereas, to do the same on Tumblr I'd have to install a third-party browser extension. And then these extensions often break when the site updates. This lack of functionality is due in part to the Tumblr staff's lack of response to user concerns. It took over two years for them to even make a block function, which is a standard feature on social websites.
dashboard: The Dashboard is Tumblr's trademark feature. It is a feed of all of the posts of Tumblogs you follow, listed in reverse chronological order. My first problem with the Dashboard is that doesn't actually provide timestamps for posts. If you follow a lot of blogs you can go hundreds of posts down your "dash" and not know how much longer you have to go until you catch up to where you left off last time you checked it. I get the sense this was an intentional design, tho, again it can be fixed by third-party applications.
Another example of a-feature-not-a-bug is the notification of likes and reblogs. When someone else interacts with your posts, that appears on your dash instead of a seperate notification area. That can be useful, if someone adds a comment, or if your post only receieves a few responses. If you make a popular post, however, your dashboard will be flooded. I made a post that went viral a few months back, and haven't had the patience to use Tumblr since. Last I checked it was closing in on 40,000 "notes" (likes or reblogs).
These two points aside, the entire format of the Dashboard itself is just "flat". Compared to the different content discovery methods on the top-right of Hubski (or subreddits, or StumbleUpon, or even a '90's webforum) the dash just feels like a content treadmill. Even with the aid of a third-party filter, there's still an inordinate number of posts that are just not interesting. So many people that post excellent stuff you won't find anywhere else also post untagged junk. For me the fandom aspect of Tumblr was what really turned me of the most. I didn't know it was possible to be tired of Harry Potter before Tumblr.
Hubski's interface and users both make it a superior website than Tumblr and Reddit when it comes to general interest topics and current events. There is an aversion, part build-in, part volitional, to trival junk. For the moment Tumblr and Reddit still beat out Hubski in regards to many niche or obscure topics. My prediction is that in the coming years the user experience of the Web will further bifurcate into the quantity-over-quality Stream services like Tumblr, Twtter, and Facebook, and carefully crowd-curated content discovery services like Hubski and /r/DepthHub .