Well, there are 'replies,' and people often use reblogging as a form of commenting. On those two counts, then, probably you'd be more interested in Posterous, which has comments and doesn't have reblogs.

However, the reblogging DOES make a huge difference. It is also really easy to use it, and they don't have ads and let you use your own domain name for free, which makes it attractive to bloggers.

(Depends also what one is comparing Tumblr to, and also, there are the network effects)