Everyone having their own definition of 'challenging' makes these discussions all the more difficult to nail down.

I consider myself an average player and while trash mobs present no challenge, bosses sometimes leave me struggling (but ultimately succeeding, sometimes one the second try after an embarassing death). That seems like a pretty good challenge line to me: not too difficult that the average player gets overwhelmed, but challenging enough that it's still fun.

If your skill is such that these bosses offer no challenge whatsoever and you can't remember the last time you messed up a mechanic, good on you and I'm sure you're killing it in savage and Ultimate raids, but I don't think that's where the challenge line for normal dungeons should be, least FFXIV suffer the same fate as Wildstar (I absolutely adored that game, so sad to see it gone )