I think it would be a handy distinction to make copying different from stealing. Copying is merely aping one or two other designs, usually visibly doing it in an inferior way. This "imitation" isn't flattery -- it's embarrassing.
Stealing is when you take elements and suggestions from five, ten or more different designs. Rather than make a cut-and-paste collage of elements, the new design fuses them into a unique new vision which may improve on each element borrowed.
To illustrate both Microsoft and Apple borrow ideas from others. Microsoft copies, Apple steals.
A great article on this concept is
Good Designers Copy, Great Designers Steal. I wouldn't even call it stealing when you improve on the ideas, you're borrowing to give something back which is greater than what was borrowed.
A good rule of thumb: If you feel compelled to ask -- you really don't need an answer. You already know what the answer is.
Kevin Airgid Uncovers a Web Design Trend: Monkey See Monkey Do. Monkey-see monkey-do is the inferior result of copying, and it is rampant in Web 2.0 style -- which rapidly devolved into
web design cliches.
It has gotten so trite and formulaic, you can eliminate the designers altogether and write up a Web 2.0 design generator script - of which there are many.
Most people copy because they admire a design and want to be admired themselves. This is a fine goal. However, when you're posted on
Pirated Sites or called a cliche, the goal is defeated.