You’d have thought I’d know this already, but I didn’t … doh !
I was doing some research into the most popular tags used. I found a WordPress support page that states that your posts will not go into the reader if they have more than 15 keywords (tags + categories). I’d been working on the assumption that more was better … appears not …
Check this out – http://en.support.wordpress.com/topics/
I’ve also realised that it’s largely pointless to put a post in category called “photography” and tag it with “photography”. One or the other. The reason for this is that the reader uses both tags and categories. Having said this you might still want to tag it as “photo”, “photos” and “photograph” for maximum exposure in the reader. As far as I can make out these are all considered to be different in the reader.
Other useful advice that I saw was to keep categories wide and tags narrow.
Why is it important to get into the reader ? The reader is the primary advantage that you have as a WordPress.com blog as opposed to self-hosted blog. The self-hosted blog’s posts don’t appear in the reader.