What does WordPress.com do well? WordPress is super customizable. A designer friend told me that 30% of the sites on the internet are built in WP! So, I love that you can do a lot with it, and there are lots of cool features. Having the ability to add all kinds of plugins is great, too, giving you pretty much endless options.
What does WordPress.com not do well? If you aren't a designer or coder and choose to build your website or blog, it's often pretty plain. Additionally, updating plugins and things is a hassle.
What do you wish you had known about WordPress.com before using? I sometimes get bogus/junk comments that have to be filtered, and as mentioned, updating plugins and things is a nuisance. As a non designer or coder, there is only so much I can do with my site if I want to keep it low cost.
Have you used a competitor to WordPress.com? If so, how does it compare? I have a Wordpress site and a Squarespace site, and I prefer the Squarespace site. The SS site looks better, there are no updates to make, it already features a lot of integrations and more. It doesn't have as many bells and whistles (ex: plugins), but still serves me well and has good customer service.