Hi Adam - we definitely see being able to mention others in comments as an important scenario, and it's in our plans to add it in the future. We're continually investigating options to bring this functionality to Planner, please stay tuned to our UserVoice for the latest updates.
The ability to follow cards would already be a good step forward. The main problem is that if you’re waiting for a task to be completed as a dependency for some other work, you only get notified in very specific scenarios.
@mention is a broad solution, but if there are other ways to more quickly address the follow/notifications rules problem, that would be a great step forward