You cannot place Google Adsense or other advertising ads on your free WordPress.com blog, write paid posts, sell links, review products, etc. Under WordPress.com's terms of service. You cannot use any kind of commercial activity on your free blog. However, you can apply for WordPress.com's advertising program “WordAds”.
2. Cannot Install Plugins
You all must have heard that there is a plugin for everything in WordPress, that is, you can use the plugin to enhance and customize your WordPress functionality. But you cannot install the plugin in WordPress.com, you can do this only in WordPress.org blog. WordPress.com comes with a limited set. Therefore, to use the plugins, you have to transfer WordPress.com to self-hosted WordPress.org.
3. Cannot Install Theme
WordPress.com provides themes to its users. But some of these themes are free and some are paid. Also, these themes come with limited options. You cannot modify or edit these themes. But if you spend $30, you can add custom CSS and fonts to your theme. However, if you want to use a powerful eCommerce theme then not available in WordPress.com. If you want a custom designed theme, you will need to migrate from WordPress.com to WordPress.org.
4. Fear of Account Suspension
WordPress.com is a very strict platform, if you violate its terms of service, then it suspends your account without warning you.