That's rather a broad question I'm afraid
The current work on the css3 specs can be found here. There is too much to document here, you will just have to read through the specs to see what's proposed and how far browsers have implemented it.
To see what can be used now you can visit [caniuse.com which shows the current state of browser support. You can learn how to use the properties [URL="http://www.css3.info/"]here and test out ideas [URL="http://css3generator.com/"]here](http://caniuse.com/).
Some css3 properties are very complicated and some like border radius are very easy. It all depends on the task in hand as some modules like the flexible box module or multi column layout are obviously complicated to an extent as they have a lot to do. Linear gradients can also be complicated with all the many options and browser variations.