What is mid-core? Defining this buzzword is a hot topic in the games industry. Here’s my favorite definition which I have honed through multiple talks with other developers.
Mid-core is a movement to create games using a balance of meaningful gameplay and broad accessibility.
Mid-core is more than a buzzword — it’s the natural evolution of games! It’s a movement. Hardcore games have some of the most memorable gameplay moments in existence but many contain large barriers of entry that seriously limit their audience. Social and casual games have the opposite problem. They are great at getting players to where the fun is and they do it as smoothly as possible, but keeping players there is difficult because the concepts are overly simplistic or exploitative. There exists something in the middle of these two extremes and this is the place mid-core aims to take us. But where exactly?
Imagine that we’re trying to find diamonds. Diamonds made of fun. It’s tiring work digging for diamonds. When you find one it’s covered in dirt and debris and trapped in rock. It’s deep underground. It’s only interesting to other miners. These people are game designers. They want to find the source of the fun. It’s hiding out there, waiting to be extracted.
A little bit of fun, fresh from the earth.
Before showing the diamond to anyone they will clean the dirt and debris off it. The diamond now shines and can be enjoyed by some people. “What an amazing find!” they’ll say. These diamonds are raw — beautiful, but rough. They are hardcore games. 
The fun is cleaned. A hardcore game.
The mid-core designer is not satisfied with the respectable audience his nugget commands. It is a niche. The nugget could be enjoyed by more if it were shaped into something. Through deliberate cutting and polishing the fun reaches its final form, a gem. 
Mid-core has brought us here.
Finally, our analogy ends with a cautionary tale. Polishing alone does not mean you end up with a breathtaking gem. Diamonds are about refraction; what matters is not only what edges you’ve removed, but also what edges you’ve decided to keep. If you don’t know when to stop you end up with, well, a see-through ball. 
It’s a facebook game.
