In low stakes PLO on an uncoordinated board, it is reasonable to always pay off top set with middle set? I remember folding middle set with no straight or flush possible only once, and I was wrong.

With no straight or flush possibilities I'd say yes in all limits. I see so many people going in with top and bottom or bottom 2 its not even funny. Unless you know they are good I'd call them.

It's fairly reasonable. And the lower limit you play the more reasonable it is. If the guy is a solid player though...who wouldn't pot-raise without a big hand or draw....you should consider mucking it on an uncoordinated board. If the guy is a lagtard and willing to do this with only 2 pair, you have to pay him off if he has it.

Make a good-sized bet/raise and fold to a max reraise. Most players at lower limits just aren't tricky enough.

I can see folding in some situations. Say you have an opponent who is VERY tight, and you are pretty certain that he will not pot-raise a limped pot pre-flop without AAxx. So he does, you call with say QQJT double-suited, flop comes AQ6 rainbow...you bet, he raises pot. I might fold that.

I just had a situation where I should have folded middle set, but I didn't. I was this close to pressing the fold button, but then I talked myself into calling at the last second.

Relevant stack sizes:
iggymcfly: ($201.36)
fitzfitz: ($100.75)

Dealt to iggymcfly: (4h, 7s, 6s, 4s)

Pre-flop:
3 calls
iggymcfly calls
1 call
fitzfitz calls
SB folds
BB checks

Flop: (Jh, 3c, 4c)
3 checks
iggymcfly bets $7.50
1 fold
fitzfitz raises to $30

Anyway, I called him down and lost $100 on that hand. If I had bottom set, I definitely would have folded there, but I just didn't quite have the discipline to lay down middle set in that spot.