I regularly monitor many different blogs on a variety of subjects. The best and most successful among them usually exhibit certain key characteristics that any aspiring blogger would do well to emulate.
Have a unifying theme. Before you begin to write, you need to know what you are writing about. Readers like to have a sense of where you are coming from and where you may be going before they decide to follow you. Your theme can be as narrow or as broad as you like, as long as it means something to you and is held together by your point of view. The joy of gluten-free living, life as a widowed mother of five, local restaurant reviews, your experiences as a graduate student, arts and crafts for kids, beautiful nature photographs — all of these are examples of subjects with plenty of potential. If someone asks you what your blog is about and you can't tell them, or if your explanation leaves them baffled, then it's time to rethink your theme.
Note that is isn't necessary to restrict every single post to your main subject. Related ideas are often a good fit and will add interest. It's also okay to occasionally write completely off-topic posts. A change of pace now and then can be refreshing for both you and your readers. But don't do it so often that you lose your focus and your fans lose interest.
Write well. By this, I don't mean that you have to churn out Pulitzer-quality prose on a daily basis. Or ever. I'm talking about clearly understandable writing that conforms to standard grammar, spelling and punctuation. Sloppy writing tells people that you don't care about what you are doing. So why should they? Bad spelling and punctuation, run-on sentences, and misused words can make your work difficult, even impossible, to understand. Readers will not spend much time trying to figure out something that doesn't seem to make sense. It's your responsibility to communicate your message clearly and professionally.
Be aware that your computer's spell-checker will not catch many of the most important mistakes, and its style-checker will tend to mislead you. If you find it too challenging to proofread your own work, get a well-read friend to help you.
Be readable. Don't let your page design interfere with your visitors' ability to read your blog. First, there needs to be a reasonable amount of color contrast between the text and the background. I am amazed at how many people think a pale yellow or light gray font on a white background is a good idea. At best, this looks like a grayed-out (i.e. unusable) area. For many people it will be nearly invisible, especially if they are over 40 or have a monitor that isn't set exactly like yours. Combinations that are too intense (lime green against black, for example) are often perceived as painful or blinding. Dark text on a light background is easiest to read. A busy background (intense photos, frilly patterns, swirly lines, etc.) can also render text unreadable. While you are choosing the colors, consider using a slightly larger font, at least 14 or 16px. If people have to squint, they will not enjoy reading your blog.
Another factor in readability is the line width. More than about 80 characters per line makes it hard for readers to track from the end of one line to the start of the next. If you do not have a wide-screen monitor, realize that many others do, and if your page simply expands to fill all available space, they will be overwhelmed by your text. Fortunately, most blogging software and hosting sites now make it easy to control the width and other design elements.
Write often. It's important to keep your blog active. If you have the energy and creativity to post every day, that's great. Otherwise, once or twice a week is enough to sustain interest. A blog that hasn't had any posts for more than a month looks abandoned. If you have to take a break, let your readers know that you will be back soon.
Make it personal. Readers respond strongly to writers who reveal something about themselves. This doesn't mean you have to expose extremely intimate, sensitive information. It just means letting people know something about who you are. Some bloggers do this with a small biography page. Others do it by occasionally using their own experiences as examples. Still others go all the way and write very openly about their daily lives. However you choose to do it, your personal touch helps readers feel more connected to you.
Caution: When you choose to reveal information about yourself, make sure you are not violating the privacy of your family and friends. When in doubt, ask them if they'd be comfortable with what you plan to write.
Be interactive. Allow readers to leave comments. Read the comments. There is no need to respond to every comment (in fact, if all you have to say is "Thanks for the comment" it gets annoying), but you should respond now and then. Readers are more likely to follow you when they feel involved in what you are doing. If you are concerned about spammers and trolls, use comment moderation. Most blogging platforms now have spam reduction tools that can be very effective.
Don't be too thin-skinned. You never need to accept obscene comments, comments that disrespect other commenters or insult you, or comments that stray too far off topic. But when moderating, try to relax and be open-minded about criticism. Dissenting opinions and alternate points of view will make your blog more interesting.
Network. Visit other blogs that are related to yours, or any blogs about subjects that interest you. Put a list of good links somewhere on your page. Support other bloggers by leaving relevant, sincere comments, and occasionally link directly to an interesting post that you then recommend or discuss. Some other bloggers will return the favor. You'll eventually find yourself part of a supportive community of people with shared interests. This will enrich your life and inspire your blog at a higher level.