I faced the same question a few days ago. I went for Tumblr in the end, mostly because of the polished feel both of the website and of individual Tumblr blogs, the low barrier to entry, and theme customisability. I've quite liked how little I've had to change to get a basic blog working how I want it (decent look and feel, Twitter feed integrated, etc).
Only nasty surprise since has been that the Tumblr founders "don't believe in comments", so there's no built-in commenting support (instead of commenting, you're supposed to sign up for your own Tumblr account and use their "reblog" feature, which isn't a decision I want to force on any readers I may attract). They do have some integration with Disqus, which I was sort of planning to use anyway, although apparently this doesn't work so well in themes other than the default "Redux". I have had a couple of problems with comments not showing up, but I think it's probably a problem with Disqus rather than with Tumblr.
I found a couple of Mashable articles comparing Posterous and Tumblr:
Why bother with comments? All they do is add noise. I like Tumblr's approach with their Likes and Reblogs: It adds a trackable story to each of your posts.
I'm a Tumblr theme designer. Would it be whoring to point you to one of my creations? I made this one specifically for people who want to write in longform; I've seen it used on a few hundred blogs now and it does a very nice job.
Tumblr is the most polished blogging platform out there. It also has built-in audience controls, meaning if you write good things, news will spread.
Posterous has an excellent feature set, but it's uglier than balls. Bad typography, bad colors, generally washed-out. If you're okay with not looking beautiful, though, it might be a better from-the-get-go choice than Tumblr. I've tried and given up on it myself.
I use Livejournal myself. Works fantastically well, best commenting system out there, support RSS, Atom and OpenID. Has a well-featured API, so you can use any number of clients to write posts.
I don't know if you can do that with posterous or not, but I know WordPress is used for a lot of sites whose content relies heavily on images. peopleofwalmart.com would be a example.
one slight turn off to posterous is that there seems to be no nice way to embed math in the same style as wordpress's latex integration. Is there such a way already or forthcoming?
edit: i've used wordpress both as a tech-dumb, just-want-to-blog-fast user, as well as set up and customized on my own server. curious what problems others have found with wordpress.
Here's my big Wordpress problem: It adds too much shit to a default install.
Sidebar? Archives? Tag lists? Blogroll? Datestamping? Comment numbers? When I install Wordpress I immediately have to go in and remove everything. It gives me so goddamn much to work with. Its themes are overall too bloated for my taste.
It's a good solution blogging-wise, but I wish they would make it easier to get rid of all the junk.
Blogger, WordPress.com, Posterous, Tumblr, Weebly, TypePad, etc... are all good choices, each with their own pluses and minuses. If you get to the point where you want more flexibility, and customization with your choice of plugins, please give Flooha a try, it's free. http://flooha.com You can upgrade later to a traditional web hosting account with control panel, ssh, email, backups, cron jobs, forwarders, statistics, file manager, and more if you like the service.
If you have questions, look me up on Google Talk as "flooha", or email me at matt [at] flooha [dot] com.
Posterous sucks because you cannot insert images where you want and you have to go through a separate interface to get your images in it. Posted one article on posterous, and don't want to post more.
If you use your e-mail program as your post editor (I use Gmail web interface), you can make it work by throwing ((nogallery)) into your title. See my recent posts for how it works: http://www.sachinagarwal.com/
Disclosure: I'm not the Sachin Agarwal that co-founded Posterous, although he's threatened to hijack my domain and redirect it to his blog. (He's kidding. Sort of. I will stab him if he does.) Because of my name, however, I get random support e-mails and have gotten pretty decent at tech support. Also, the Help section is actually rather well documented if you know what you're looking for.
I wanted an easy to admin wordpress site and picked dreamhost.com as its just about as turn-key as I needed. Backups, upgrades all work seamlessly. This gave me the added benefit that for a fixed price per year, I can host other people's wordpress sites on the same plan. Turns out I have several friends that want a site but know nothing about running one, so I do it for them.
If you're looking for ease-of-use, posterous or Blogger. I'd probably go with Blogger, but there's nothing wrong with posterous either.
If you're looking to eventually do "cooler" stuff with blogging, go with WordPress or MoveableType. I've been with MT for almost five years. If I had to do it over again, I'd probably stay, but WordPress is looking awful good as well.
But for now? Just kicking around? Make it as easy as possible on yourself.
I'm writing a new blogging platform that is definitely much simpler than Blogger, Posterous or Tumblr. I'm hoping to get my first iteration out by the end of the week (yay for Thanksgiving break!) and will post it on Hacker News. But I couldn't find an email on your account so feel free to email me - abii @st anford.edu
I've heard great things about squarespace (http://www.squarespace.com), though I haven't used it myself.
They seem to have import/export functionality to all the other big blog platforms, so it wouldn't be hard to jump out to another platform if you wanted to.
when would that be useful? people usually have a single blog rather than duplicates at different domains...otherwise your users and comments are fragmented.
furthermore, the features are in the interfaces for writing and reading posts. you might as well choose a platform that allows you to write posts with all the image floating and code snippets you want. or whatever.
I went with tumbler for mine, the only thing I don't like is how it links to the actual post. Instead of linking the post title, like every single other blog on the planet, you have to scroll all the way down and click the "Posted 4 days ago" link
Only nasty surprise since has been that the Tumblr founders "don't believe in comments", so there's no built-in commenting support (instead of commenting, you're supposed to sign up for your own Tumblr account and use their "reblog" feature, which isn't a decision I want to force on any readers I may attract). They do have some integration with Disqus, which I was sort of planning to use anyway, although apparently this doesn't work so well in themes other than the default "Redux". I have had a couple of problems with comments not showing up, but I think it's probably a problem with Disqus rather than with Tumblr.
I found a couple of Mashable articles comparing Posterous and Tumblr:
http://mashable.com/2009/06/29/posterous-vs-tumblr/
http://mashable.com/2009/09/28/tumblr-vs-posterous/