Reminds me of a situation here locally with a sandwich shop...this shop was in business for many decades. The line each day went around the block. The owner finally retired and closed shop - he didn't want anyone to take over and possibly make lesser products - didn't want his name associated with it.
Well, some of the employees decided to open a shop (similar name) since they already knew how to make those sandwiches...there are whisperings of legal problems - why? not because they made a similar sandwich, but because they implied that it was the same and only the name had changed.
I agree with the public domain issues of traditional or published patterns. I think a lot of the concerns have to do with whether you are intimating that it is your original design. Often you have to sign a declaration stating whether it is an original design or if you have permission to use it - especially if it being published. If you use other ideas for your project and give credit where it is due (or preferably get permission)you are usually ok - after all, it is almost never an exact duplicate! But each individual needs to decide for themselves where their artistic ethics lie....