This is going to be a short and sweet blog post. If you remember in Civil 3D we had two options in Create Design pane of the Home ribbon-Create Corridor and Create Simple Corridor. Now Create Simple Corridor is removed and it is all combined into one and only: Create Corridor.
If you also remember how in Civil 3D 2012 and earlier, we had to invoke this command, then pick our Alignment, Profile and Assembly before we get into a Create Corridor where we needed to do other things such as targeting, selecting style, etc. In Civil 3D 2013 it is all combined in one dialog which now looks like this.
Now in the same dialog you can see and choose your Alignment, see all your Profiles and Assemblies and even Target your surface.
Other difference is the dialog that follows (see second image). Look at the picture and notice that the dialog includes station frequency, selecting regions from drawing and lock regions. More about this you may be able to read on Being Civil in the nearest future.