Few mistakes that I see new bloggers make ( including me ) and if you recognize them try to avoid them from now on.
Blogging for Adsense
This is one of the biggest mistake many blogger starts with. First they get the Blog out and the next thing they do is put Adsense ads all over the place and make it look cluttered with Ads. I have seen many blogs having maximum number of allowed Adsense Ads, one after the other thinking more ads fetch more money but actually thats not true. You should realize that its the user who fetches you the money and not Adsense. I can rest assure you how many Ad units you have on your page but if its only you who visit that page then you would make $0.00 every time you see your Adsense account.
Copying from here and there
The other big mistake newbie blogger tend to make is copy the content from other websites and post them. This is many fold a mistake. First its your blog and so it should be your views and content and next is copied content is a kill from Search Engine traffic point of view.
Blogspot / WordPress Blogs
Yes this is also a mistake. Its always good to experiment and then get into reality and trying your hands on blogspot / wordpress for some experimentation is good but if you are serious about your blog I am sure you can afford a domain name and hosting which are as less as $5 per month to start with. If you can find cheaper than this start with it or else try JustHost / hostgator / GoDaddy. Checkout my experience and review of some of the Best Cheap Shared Hosting providers.
I have seen on many forums where people feel proud on the number of blogs they own. The number of websites they own ranges from few tens to hundreds and I am sure if there are few blogs in them it would be dead. If you have a blog I think you should continuously update it with fresh content. People tend to hate if they do not see anything in the RSS Reader for your blog feed
Neglecting User Comments
I have posted so many comments on issues I see on blogs I visited but no one responded to the comments and I was probably the only commenter for many days. If the blog is new, comments would be less but atleast the author should read and reply. Other advantages is this comments help you get more topics. On my finance blog see this comment by one of the user which resulted in a new post for me.
If you have some more to add do so in comments.