Why I've gone to Posterous
I've been thinking about this for a while, and finally decided that Tumblr just isn't what I need.
Don't get me wrong, the concept of Tumblr is brilliant. Different post types depending on what it is you want to post, each styled differently, and all easily postable. Sadly, whenever I got the urge to write a post, I saw the 'Link' option, added a couple of sentences to the description and boom, I've blogged, let's have a smoke break.
It made blogging so easy that I never actually blogged, strange as it sounds. I just fooled myself into thinking I was blogging.

Tumblr also seemed to focus too much on the presentation. There are some gorgeous Tumblr themes. So gorgeous, in fact, that I spent more time playing around with the theming API than I ever did posting. Compared to now, where I've got a pretty basic look-at-all-the-words theme that I've thrown together in an evening, and you can hopefully see the focus is on the content, not the packaging.
I came across this article that seems to sum up the form vs function argument pretty well, but I think he gets the conclusion wrong. He says Tumblr is beating Posterous because it's better designed, and I suppose he's right, but that's also why I didn't like it. It was so close to being good, but it just had a bit too much Twitter and bit too little blog. You can create a post by clicking one button. Tumblr will detect what kind of page you're on and create a new post that matches, so video for Youtube, photo for Flickr, etc.
I don't think creativity is something we should be simplifying, and that's my problem with Tumblr. I don't like the emphasis on reblogging (which is basically just posting someone else's content with an 'I agree!' or 'Me too!' attached) instead of creating new and interesting things to read or look at.
I wanted to get away from the complications of a full Wordpress style blogging system, but Tumblr just went too far in the other direction. Posterous seem to have got it just right, which is why I'll hopefully have that yellow tab in the top right for quite some time.
Switching to Tumblr
I've been playing around with various different blog platforms over the past week or so to find the one that works best for me.
I used Sweetcron for a while, and even went so far as to build my own theme for it. Then I decided I didn't like it, for a lot of the same reasons the Sweetcron creator doesn't like it. Here's my own reasoning.
Sweetcron is great in a 'fire-and-forget' kind of way. You hand it a bunch of feeds from various websites, like Flickr and Delicious, and it makes them into a nice little blog-like stream of all your activity.
This is where sweetcron started to fall down for me. I normally use Instapaper to queue up stuff to read and then binge on it over the course of an hour or so. This meant that a lot of links and shared RSS items would show up at once.
I had the same problem with Flickr, each photo got its own item, so a new set would fill up the first 4 or 5 pages of the stream.
I use different tags on different services, so even though Sweetcron pulled them all into one big bag o' tags, they still weren't much use.

All of this meant that I started to worry everytime I added something to any of the sites that Sweetcron fed from. I figured that if I was going to worry that much every time I posted something then I might as well use a separate blog (or Tumblog in this case) and just post to it when I wanted to.
So there we have it. By solving my problems with blogging, Sweetcron proved that I didn't need it at all, that all I was doing is using Flickr, Delicious, Twitter and others as a sort of cross-domain tumblog. So I switched to Tumblr. It's fun, simple, quick, easy to theme, and it has an iPhone app
Update: Well, I'm never happy, since I've now left Tumblr at the altar for Posterous.