Depends entirely what you mean by copy.<p>Here are 3 very common scenarios and likely outcomes.<p>Copy verbatim is really frowned up. Stealing an image outright is seriously frowned upon (you get served a DMCA notice or similar), same with well crafted and very specific CSS / HTML. (No one cares about layout however, steal all you want)<p>But what happens when you are inspired by someone else's work? Did you take something good and make it better! Great we love that; this form of appropriation is typically considered flattering. (you get featured in a blog)<p>Or<p>Did you take something that looks awesome and re-purpose it for something that it does not quite work for, and when stitched together the design fell apart? perhaps because it was refactored by the hands of someone inexperienced? thats when people get really mad and they start coming after you.
(You get made fun of in a blog if the changes or substantial enough to be called 30%)<p>Its worth a long conversation to be honest, this was the shortest unedited 2 cents I could post on the subject without reverting to well known cliches.
Blatant lifting of their css or html is socially unacceptable and may break copyright law.<p>Deriving your own ideas from someone else's design is typical. Everything is derivative. Everything is a remix.
Pinterest's design wasn't original either, see the jQuery Masonry plugin circia 2009 (<a href="http://masonry.desandro.com/" rel="nofollow">http://masonry.desandro.com/</a>). However, a verbatim copy of their layout will definitely be noticed - think about your use case seriously and tailor the design to match your requirements.