"How do you think the campaign is going?"
"What did you like the best so far? Which part was the most fun?"
"Anything that you wish was not there because it was no fun for you?"
"any parts you think we need to improve? (focusing as a group, taking time doing nothing, etc., grinding to a halt for looking up rules, arbitrary rulings that go against the rules, giving everyone a spotlight chance, etc.)
"Combats going well and fun or are they too slow or too mechanical or whatnot?"
"Enough talking and politics? Too much?"
"What did you think of the puzzles/traps etc. that came up?"
"Tone of the game fun? Is it exciting and cool or more dark and apocalyptic than you want for D&D?"