Of course Posterous is just another blogging platform. Have they tried to be anything else? The way you described how you use tumblr does seem to suggest that Posterous is actually a better fit, but it appears that you just found tumblr first and didn't bother to switch.

The Posterous switch campaign awhile back was targeted at users like you. That's how they were trying to set themselves apart. It might not have worked well, but the intention is clear.

There is only so much a utility product can do to diversify itself from competitors. A condom is a condom is a condom. As long as it doesn't leak, it works. Would you complain that Trojan's products are not sufficiently different from Durex?