There are a lot of different factors that make up a successful blog. If you want to know what some of these factors may be so that you can work towards them, you've clicked on the right blog post!
Traffic
The first and foremost factor is traffic. It's absolutely vital for a blog to have traffic because traffic: gives your blog a purpose, increases your ratings, your viewers who make up your traffic spread your blog by word of mouth, spread the news about your latest work, recommend your blog to people who might like it, etc. so on and so forth. Basically, traffic is one of those self-replicating things meaning that if you don't have traffic to start with, you won't get any more. Getting traffic is a relatively simple process and if you want to look into it more, check out some of my other blog posts.
Marketing
How you market your blog is very important to and there's many ways to do it but people primarily market their blogs by spreading the link around and getting high-traffic, high-value keywords involved to increase the value of the traffic coming your way. The best way of marketing your blog is to do a little experiment with the Google AdWords campaign system to find out the value of a keyword that people type in and find your blog with. Using this process, you can easily find high value keywords that relate to your blog.
Content
This is quite easily related to the 'traffic' factor since bad content = bad traffic ratings. Basically, if your blog is full of good, high-quality content of a respectable amount with new content being published daily, you can expect higher and higher traffic ratings by the day, until the one fateful day you might hit a Traffic Drought (see my previous post of Traffic Droughts and how to beat them).
Design
Your blog should have a great, eye-catching design that attracts customers and appeals to their 'pretty colours!' side. Everyone has one, you know. Just make sure that the design, while eye-catching, also relates to the idea of your blog. If it's a business blog then you'll want some impersonal, grey/black design that emits an aura of professionalism whereas a niche blog should have a design that pertains to the niche of the blog itself (i.e. a Writer's Blog could have something like a pen lying across a paper as the background image).
Interactivity
Your readers should feel like your interacting with them through your blog posts - include things like puns, comedic sarcasm and hilarious comments here and there to make your blog posts feel less impersonal and make your readers feel like your right there in real life, talking directly at them. Create polls, forums, special weekly/monthly events, etc. to make your readers feel like they're interacting with you and that your blog is more than just a place to go, find out something and then try it out. It should be an awesome experience for everyone who visits. My opinion on interactivity is to build a community with your readers over something like forums.
Well, there you have it! The most important factors that contribute to making your blog a success. Of course, there are more but these are the ones you should work on the most to make sure your readers the best experience out of your blog and to make sure all your hard work pays off and you have some enthusiastic readers to communicate with. Honestly, once you build the bridge to communication with your readers and your traverse it, you'll find they can provide you with a lot of great ideas and helpful insight, no matter how experienced you are!