Mmmm... this was such a wonderful Thursday puzzle, just the antidote to a crazy crazy day at work. That plus a nice glass of wine, don't you know.
60A: Backtracking ... or what 170, 27- and 46-Across are doing? (REVERSINGCOURSE) explains the clever stuff that's going on in the other three long clues. Three different examples of "courses" are placed in the grid backwards. Thus 17A: Home of the Masters (that is, Augusta National) shows up in the grid as LANOITANATSUGUA. Do you know what's really crazy here? I had the ____ANAT____, and unfortunately, the "...ional" fit in the remaining squares in the standard direction! This set me back for a while.
The other two "courses" are pre-algebra (27A: Something unknowns are introduced in - ARBEGLAERP - I love that clue), and appetizers (46A: Starters - SREZITEPPA). Such a great idea, and so well executed.
But you get lovely additions, as is to be expected with Ms. Burnikel's work. GONDOLA... PICANTE... SHREDS. There's the outstanding 3D: "Easy! Everything will be O.K." (DONTPANIC).
I really loved the clue at 11D: Nonmonetary donation (ORGAN). Also 68A: What bagpipes are often played in (KILTS) - not a musical key. Which by the way is typically in the key of OH MY GOD THAT'S SO FREAKING LOUD.
The pièce de resistance was AUNATUREL crossing LEISURE. And that clue! 39A: It's not working. Hah!