.com has this feature .org does not have this feature
In the user profile section the .com version has a whole slew of options for something called proofreading. The .org version has none of these.
Proofreading is a more powerful tool than just the normal spell checking which comes with both versions. There are about 10 options for WordPress to check for grammar and content problems, such as clichés and double negatives. There are further options to automatically proofread the post either on any update or on initial publishing. In general pretty powerful stuff. This may or may not be a worthwhile feature for you, especially if you develop your post off-line in a different tool. The differences between the two seem to be the After The Deadline plugin which can be added to the .org version. This plugin requires an API key obtained after registration, which is possibly the reason why it is not automatically included in the .org version. The plugin is free for personal use at this time according to the website. Here’s a link to the WordPress support page for this feature.