I think it's important at least initially to have a specific subject. Then you get readers who share the same interest. After awhile you will probably end up blogging about other things too but by then your readers will have decided whether they like your writing or not & - I think - if they're still there they will be happy reading non subject specific posts. I've even published two books since blogging- one on the main subject of my blog (World of Warcraft) & one a sci fi book on parallel worlds- I'm hoping my followers will like both!!
I think the content has to be interesting, well written, and targeted to a certain audience demographic. I think a touch of humour and light heartedness also helps. Sure, it can be common to all but be sure your blog is not too all over the place. Certain interests will attract specific readers.
Most of all be honest. Be yourself. Be original. Have fun with it.