It seems appropriate today to give one of the possible five LETTERGRADES to 1A, as is the custom of my esteemed co-bloggers. I think FISH merits a C. :)
I like the theme, but truth be told, I didn't guess any of the theme answers from the clues. I got them all off the downs - always a risk on a Monday, I suppose. The theme clues, all groups of letters having something in common that is revealed by the answer, bring to mind one of my favorite puzzle types from old Games Magazine like 88 K on a P. Remember those? 39A's theme answer (CHEMICALSYMBOLS) is smack dab in the middle of the puzzle and goes edge to edge, which I think is very nice. Want to hear a joke about potassium? K.
I was spoiled for choice when it came to picking a favorite today. My first love was OHHI at 14A. When unpunctuated, it seems so casual. Horace and I used it all day during our Internet communications. Another strong contender is SHOTTOHELL at 3D (Utterly ruined, informally). And, last in this category, but not least, is the perennial favorite: EEL.
Other clues I liked were
OER - as the opposite of 'neath (27A)
TEEM - so evocative of too many unpleasant somethings (24D)
SASS - (32D)
TILE - for the reference to Scrabble (tm) (31D)
ECO - as a clued combo with freak ( )
There wasn't much of what we traditionally call Huygens material in this grid, but plenty of answers I think Huygens will have enjoyed including FERMI at 21D, (all the) MEATS at 41D, and BALI at 42D.
My least favorite answer in today's puzzle was the clue at 1D Centrally located for FOCAL. It didn't feel APT.
Overall, I thought the puzzle was AOK.