Tumblr has an interesting take on blogging.
First, and most important, tumblelogs are cleaner and uncluttered to read. I’m turned off by all the clutter on the top and on the side in Blogger (disclosure: I work for Google), WordPress and Google+ . It distracts the reader from reading the post, rather than focusing his attention on it. Here are screenshots from these sites, with the stuff other than the actual content of the post highlighted in red.
Blogger has a toolbar on the top and all the junk on the right:
WordPress is worse: it has navigation on the top, a search box on the top-right, a useless big image on top, and more junk to the left of the actual blog post. Notice also how the actual blog post can begin only towards the bottom of the screen:
Notice how clean Tumblr is: it has only a border on the top, and a few things to the top-right:
G+ is slathered in whitespace for some reason — there’s lots on the top and to the right of the post, forcing the post into a small area. The whitespace also looks random and awkward, subtracting from rather than highlighting the content:
Tumblr is the best blogging tool I’ve used.
In addition to being uncluttered for readers to read, Tumblr is also cleaner and uncluttered for me to post. This is, strictly speaking, not as important — I’d use a blogging tool that has a worse UX while composing a post if it results in a better experience for readers. Which is, after all, the purpose of a blog.
But I find that a service that makes posting easier makes me post more. This is, at one level, just the cost-benefit tradeoff — if I really care about something, I will post even if there’s a little more friction involved in the process. But a lot of times, I don’t deeply care about something, and I’m on fence regarding whether to bother posting it, in which case I find myself posting if I have a frictionless way to do so. John Gruber  describes this eloquently in Untitled Document Syndrome.
In addition to being uncluttered, both for the reader and the writer, Tumblr lets you follow blogs within Tumblr itself. This is interesting. It makes for a more integrated platform, where I don’t have to use a third-party reader to read the posts. I much prefer things to be integrated rather than having to sign up for disparate tools and then having to plumb them together using various pieces of glue. To follow a blog in the traditional world, the user has to learn what RSS is and how it works, research the available feed readers and pick one, create an account there, install its apps, login everywhere, and so on . This was a significant hurdle back in the day even for a geek like me. I think it’s safe to say that RSS plain doesn’t work for the median user. RSS is like a command-line interface — broken for most users. While Tumblr, in that analogy, is the GUI — a simple, no-friction way of following an interesting blog right in that web page itself. Just like the follow button and create an account and you’re done. No need to bother about RSS, research and pick an RSS reader, figure out how to subscribe to a blog in that reader, and so forth. Just point and click. Command-line vs GUI .
Reblogging is another interesting idea. Reblogging helps spread the good stuff. Traditional blogs don’t handle this well — if I find a post interesting, I want readers of my post to read it. I can link to it, but that’s a level of indirection, and whenever you have indirection, a lot of people don’t follow the indirection.
Reblogging also ends up giving appreciation to the writer. This is, of course, important in any creative field. The writer is human, after all, and whether people care about what he writes is important to him.
Taken together, all these aspects — a clean reading experience without distraction, easy posting, an integrated experience where you follow blogs within Tumblr itself, and reblogging — result in a system that does enough things differently from the usual blogging tools that it’s interesting. Most blogging tools are built on exactly the same ideas copied from each other, while Tumblr is a genuinely different, and interesting, take on the problem. The word is overused, but it’s accurate to call Tumblr innovative.
Unfortunately, it’s still not as easy to post something on Tumblr as it is on G+, where you can post something publicly, not just among the social network. That’s a tremendous accomplishment for G+, since it wasn’t designed to a lightweight blogging tool. In fact, in that sense, G+ is the best of both worlds — my posts go to both my friends and to the world at large, as with a traditional blog. Granted, more of my friends are on Facebook than on G+, but almost zero of them are on Tumblr.
So, Tumblr has the best reading experience, and is therefore the best option, if I have to pick one, but G+’s ease of posting matters.
 Why do I mention Google+ here? Because it’s a good lightweight blogging tool.
 Not that I agree with him much anymore, but that’s a different topic.
 And repeat the process all over again when the service you use is turned down.
 I’m not saying that choice and openness and interoperability don’t matter. They do, though they are secondary — tools most users can figure out how to use are more important than tools that are open, interoperable, or chocolate-covered. Notice that I said “secondary”, not “irrelevant”. It’s not one or the other, and we need both, to cater to different kinds of users, as long as we remember who the majority of users are. In any case, Tumblr provides an RSS feed for power users, while making it easy for normal users to follow a blog within Tumblr itself.