It appears from LC's account structure that users with Free and Individual accounts cannot add Comments to a diagram – but those with Team and Enterprise accounts can. The implementation of this structure has created an enormous amount of confusion for my team and our clients!
1. As discussed in other threads, this lack of functionality for occasional users – and the expectation for them to pay – creates a severe limitation and makes LC much less useful.
2. Providing the option to share a diagram with Comment access with a user with a Free or Individual is misleading for the person sharing. Similar to the way Google Docs lets the sharer know that the recipient doesn't have a Google account at the time of sharing – it would be helpful for LC to inform the sharer at the time of sharing that the recipient will not be able to comment given their account type – and switch the access to View.
3. Alternatively, it would be helpful to either: 1) inform the recipient that – while their status shows as Commenter in the upper right – they can't comment given their account type OR simply display their access as View; and 2) not allow them to right click on an object and choose Comment, only to then pop up a blank "Comment" box with no way to enter text and no explanation as to why.
4. Lastly, while a recipient with a Free or Individual account can't add an initial Comment, they can view existing comments AND reply to those Comments – creating more confusion about the access!