What separates a successful blog from an unsuccessful one?
First of all, I would say it takes commitment. If you assume that there are enough readers out there to go around (millions of them), then it is likely you will find an audience if you hang in there long enough.
I know from my own experience that I have simply not been "committed" to blogging. With all the things I have to do in a day, I just have not been convinced it is worth spending hours becoming a professional blogger.
I also have no doubt (or at least, little doubt) that bloggers with useful things to say can gain an audience and a certain notoriety, and possibly even earn a living doing blogging. But they have to be committed writers, thinkers, researchers and commentators, and not many of us fall into that category.
But having said that, I think someone who is committed to blogging has at least as much of a chance of becoming a successful blogger as does someone committed to becoming a successful entertainer, politician, musician, artist, baseball or tennis player.
It's just that very few of us think of blogging as a vocation, and until we do it is unlikely that it will become one.

Stumble it!
My top tips for making your blog succesful are:
1. Choose an interesting niche and keep your blog focused on it.
2. Write engaging content on a regular basis.
3. Add things to your blog that will encourage people to return: surveys, contests, free reports, etc.
3. Build inbound links from sources that are likely to generate traffic.
This last one is big. Without traffic your blog is nothing. You can get traffic if your content is any good.
Build links by joining forums, commenting on related blogs, joining relevant community groups (Facebook, MyBlogLog, MySpace, LinkedIn, Twitter, etc.), forming alliances with friends and acquaintances, write articles - do as much of this as you possibly can.