User Sessions
The easiest to track metrics on your blog is a simple number of user sessions. Not only is it easy but it gives you a good understanding of the size of your audience. Any stats package can easily track user sessions.
RSS Subscriptions
Who is subscribed to your blog? This is always good to know as RSS subscribers tend to have a vested interest in your blog moreso than who is casually reading a single post. In a way you can think of RSS subscriptions as your "hardcore" readership.
The easiest way to track RSS subscriptions is to track unique visitors to your RSS feed. Note that unique visitors are different from user sessions. Unique visitors counts a person once, where as user sessions counts the number of times that user has accessed your RSS feed. Given that we're looking for how many different people are accessing your RSS feed, using unique visitors gives us a more accurate number.
The Problem with RSS Subscriptions and Aggregators
While a count of unique visitors to your RSS feed will give you a good idea of how many people are subscribed to your RSS feed, it doesn't tell the whole story. RSS aggregators like Google Reader work in such a way that the aggregator itself grabs your RSS feed, not each individual user subscribed through the service.
What happens is that if, say, 200 people are subscribed through Google Reader (or a similar service like Bloglines, NewsGator, etc), those 200 people are only counted as one unique visitor to your website. The more popular your site, obviously the more subscribers you miss this way.
Number of RSS subscribers for the Walker NMI blog through Bloglines. Not counted in unique visits to our RSS feed.
Number of RSS subcribers to Kottke.org through Bloglines. The more popular the blog, the more subscribers you miss.
So how to get around it? Thankfully most major RSS aggregation sites add a subscriber number to their user agent string when they access your RSS feeds. For example, here's Google Reader's:
User-Agent: Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; 4 subscribers; feed-id=1794595805790851116)
Notice the bolded part. This will tell you how many subscribers are actually subscribed to your RSS feed from Google Reader in this example. The key here is to parse your log files for the various RSS aggregators out there for this extra info and add it to your unique visitor counts on your RSS feed. This will give a more accurate number of RSS subscribers (see more info in Google's FAQ).
Trends (over time and vs other websites)
A high number of user sessions and RSS subscriptions is great, but what about trends over time? Is your readership growing? Trends over time are perhaps the most important metric of all "simple" stats because it shows relative success (or failure) compared to yourself and perhaps other websites.
A blog with 10,000 user sessions per month growing at 30% each month in many ways is more successful than one with 20,000 user sessions only growing at 3% each month. Momentum does have an inpact, and is a key metric to rate your blogs (or any website) by. This is especially true because web stats by their very nature are imperfect. But as long as the way you collect your web stats remains consistant, trends up or down will tell you a lot as to how your website is doing.
Examples
Walker Blogs user sessions shown at MW 06?:
Walker Blogs user sessions Lifetime to Date:
Walker Blogs user sessions compared to the top 3 Walker websites:
Not only are user sessions gaining, but the trends over time have been fairly a fairly consistant 20-30% growth every month. Amazing growth compared to other areas of our website.