K2 is really nothing special on its own.
All it does is that it allows you to add extra fields to its own articles called items (which is something that should exist in joomla on its own, but no, they're more concerned about making a responsive back-end, like someone is gonna run their website from a mobile).
Now, when you couple that with a good filter for k2 and a related items filter, you get something.
But its also missing guest article submission with extra feilds (the one for reg users exists but sucks) that an admin could approve and it would appear in selected category.
All these things are simple and should be in Joomla core but they're not, which keeps joomla from being a great CMS.