One change to what I've said so far:
I'm all in favor of riddles/puzzles that are an alternative to a fight, as long as there's no clear 'path of least resistance' that is much easier.
As an example, Cabal for One (ok that's traps instead of a puzzle but you get the idea) - you can choose a harder boss fight or you can choose to have lots of traps. Below elite, neither option is clearly 'harder' or 'easier'. People do what they prefer in there. (Mostly people run the harder boss way because on elite that is the clear path of least resistance but on other difficulties the balance is right).