OK, I'm gonna make one or two friends a little upset here, but I keep seeing them make some very critical blogging mistakes and I wanted to enumerate them before I'm the only blogger left with any readers and Google juice [yes I'm stupid, check my last name]. Let's jump right in...

1. Forfeiting Google Juice
   The worst mistake you can make is to forfeit your Google juice. Remember all those people linking to you? Well, you move your blog entries and don't properly redirect the Google juice to your new blog, then oops, you lose. Admittedly, I once did exactly this big mistake, but that was three years ago and before I knew what Google juice was or that it was valuable. I've had countless friends on my blogroll make that same mistake since. Let's hope this prevents any future recurrence. There are many ways of properly redirecting your Google juice; you could simply leave your old posts in place and put an h1 link back to your new blogsite with the text My New Blog is Over Here or something like that. And don't forget to point all your autodiscovery code and RSS feeds to the new blog. A more advanced technique for moving blog entries involves an HTTP 301 permanent redirect. If you don't understand what I mean by 301, then don't bother trying it, you're likely to cause more damage than good.
2. Forfeiting Your RSS Feed
   An all too often mistake is forfeiting your RSS feed. It's very likely that someday you will move your blog. Redirecting the RSS feed is often a nightmare, if not impossible. Your best bet is to use FeedBurner from day one.This way you can move your RSS feed with you, wherever you blog.
3. Invalid RSS Feeds
   I've known quite a few bloggers who's RSS feeds were broken for months on end. Now, don't get me wrong, even my RSS feed is broken from time-to-time. Software has bugs. You can prevent your feeds from breaking by simply subscribing to your feed in various RSS readers (especially the one you use) and double checking from time-to-time that you are getting stuff.
4. Making it Difficult to Subscribe
   Many bloggers don't make it easy enough for their readers to subscribe to their RSS feeds. If you don't have RSS auto-discovery, then you are losing readers. If you don't have an visible and obvious link to your RSS feed, then you are losing readers. If you don't have an email subscription form like Rmail, FeedBurner or FeedBlitz, then you are losing readers. Also, make certain your email subscription form is working. Subscribe to your own feed and double check that you are getting a regular email. Most RSS to email services don't work reliably.
5. Blocking Your Readers
   With the advent of splogs, I've seen an increasing number of users who are actually blocking content from been served. A very common instance of late is blocking images when the referrer is not your website. This means you are blocking all kinds of Web-based RSS reader, including email-based RSS reading. I've heard a couple excuses for why people do this and I don't buy any of them. The best excuse is that you don't want another site hotlinking to your images. My response is to upload them to Flickr. But then you don't control your images? Unless you are the next Picasso, I don't think that should be a problem.
6. Sucking Up to A-listers
   This is likely the biggest problem most new bloggers face. The thought is that if you link to a lot of a-listers, that you'll accumulate a lot of referrers and readers. Well, it's true if they actually link back to you, but most a-listers have 100 people linking to them daily and it's highly unlikely your post will catch their eye. Instead, try linking to a broader range of bloggers. When you link to a d-list blogger, you're likely to gain a reader for life and several dozen links in return. I'm the ultimate d-lister and I make certain to return all Google juice in spades.
7. Not Reading Your Readers
   If a reader is leaving regular comments on your blog, then subscribe to his blog, find out more about him, so you can engage with him more completely. If you comment on your readers blog, then there's a greater likelihood you are getting a reader for life.
8. Accepting Trolls
   If you have a belligerent troll, then your best bet is to encourage him to leave. In fact, I recommend deleting his comments to discourage him. I've even played with some trolls, discovering their IP address and forwarding their requests to other rather nasty websites. After one or two visits, they got the hint.
9. Putting Yourself on a Pedestal
   This problem doesn't happen too often, but from time-to-time a-list and b-list bloggers lose control to their egos. They start posting about how great they are and commenting negatively about their own readers. I'm not sure if that works for other people, but I'm pretty quick to unsubscribe when a bloggers puts himself on a pedestal. If you are already heading down this path, then simply posts a few entries that make fun of yourself on a regular basis to keep yourself grounded and off that pedestal
10. Partial Feeds
    Full versus partial feeds has become a religious issue in the blogosphere. My argument is simple; the more words you inject into an RSS search engine, the more referrers you'll get. Some of those referrers will become readers and/or link back to you. End of story.

Bonus: This one I fail todo all the time. Not using a spellchecker and a grammar checker. In fact, I did it again. Ascribe it to laziness.