How focussed do you think websites should be kept, and when is it useful to have a realted site with it's own domain? I see pros and cons to both, but would love to hear soem experiences with this.
TBH there is nothing wrong with subdomains, I started with them and they were the best thing for me at the time being. But now I have my site, and stick to it. Great experiences with both, though.
I tend to use a sub-domain as a major divider. I will use it for something that is loosely related to the main site. Often times I might use them as redirects and cnames for other sites related to my subject as well.
Otherwise I will use subdirectories.
One thing to remember is that Google Page Rank does not get shared across subdomains. Generally, sub-domains are considered to be different websites.
So really the decision to go with a sub or top domain may be whether you need people to remember your address, it sounds like?
I, as well, do not think that sub-domains should be overlooked. You should just think of them as "folders" that could help classify things from your main website.