After Alt+S scaling, I can't get the bottom to look like in the video at 12:55. It seems that the extruded faces are scaling not as intended. What am I doing wrong?
You didn't make and separate (with Y) the planks.
Now you are Extruding and Scaling a 'box' and this is the behavior you'd expect.
When you first separate the sides, you'd have (in your case) 5 individual faces that would scale like in the video.
I'd highly recommend watching https://cgcookie.com/course/fundamentals-of-3d-mesh-modeling-in-blender
I had the same problem. No matter what I did, the faces extruded down and in at a 45 degree angle. Finally went old school. With the entire mesh selected, hit Alt and then E to bring up the Extrude Menu. Select Extrude Along Noramls and then move mouse to extrude faces in the appropriate direction. Tried to post this as a question but the site wouldn't let me. The chest on the right uses Alt + S. The chest on the left uses Alt + E, Extrude Along Normals.
Thank you! I fast-forwarded through the video and missed the part (starts at 11:30) where Kent clearly explains how to separate all the planks :)
I just wanted to say that the "extrude along normals" option under extrude in the tool menu is what I've always used. I really like the yellow handle that you pull on. Then I always set the extrusion depth to be some even number in the operations box.