All runtimes support all Spine editor features where the game toolkit allows it. Love and Corona currently don't render meshes. Unity has the most features (thanks to Mitch!). The runtime for Unity can interact with Unity GameObjects, eg to have parts of the skeleton driven by physics. This can be done in other runtimes, but you'll have to implement it. It isn't super hard, but in Unity you can just click the mouse around to do it. 🙂
This shows how easy it is to setup a cape to be controlled by Unity physics:
https://www.youtube.com/watch?v=RFCpqeW2FHA
This shows Spine's IK keeping feet from penetrating the ground as well as keeping the hands on the sword, plus the cape physics:
https://www.youtube.com/watch?v=_S22WIWOp7c#t=174
Here's a (very strange) Unity web demo of IK:
http://www.xdtech.net/spine/bawk.html
It was inspired by this forum post about chickens and IK:
viewtopic.php?f=8&t=2753&p=19144
Spine works with normal mapping to give realistic lighting for skeletons, taking into account bone rotation so lighting is correct for any skeleton pose. This can make skeletons look very 3D.
https://www.youtube.com/watch?v=QrlbYXKR_OA
Our Spine Unity runtime also can do "baking", where it converts a Spine skeleton into a bunch of native Unity scene graph objects, plus native animations for those objects. This allows you to use Spine for rigging and animation, then convert to Unity game objects for use with other Unity features. This video is a bit long, but shows it if you are interested:
https://www.youtube.com/watch?v=sCBnFK9PEmU
Finally, Spine skeleton animations can be used with Unity's Mecanim, which is their animation system with a state machine, blend trees, etc.
