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совсем другое дело stresses
gen.	another pair of shoes; another cup of tea; that's another pair of shoes (Anglophile); horse of a different color (Errandir); quite a different proposition (Anglophile); it makes all the difference in the world (Anglophile); quite a different story (something is quite a different story lexicographer); horse of a different colour (Дмитрий_Р); a horse of another color; game-changer (Bullfinch); a totally different affair (Don Quixote); a whole other story (VLZ_58); a whole new thing (suburbian); another matter altogether (A.Rezvov)
Gruzovik	quite another matter
amer.	new ball game, whole new ball game (Выражение из бейсбола marina_aid)
idiom.	a whole new ball game (It's one thing to babysit your friends' kids from time to time, but having your own children is a whole new ball game. Drozdova); an apple off another tree (Bobrovska); a different kettle of fish (dict.cc Andrey Truhachev)
idiom., amer.	different kettle of fish altogether (Of course their economic policy is a different kettle of fish altogether.)
inf.	a different bunch of coconuts (Wakeful dormouse)
math.	is another thing altogether
совсем другое дело!
gen.	that's more like it! (Bullfinch)

совсем другое дело: 34 phrases in 6 subjects

Cliche / convention	2
General	19
Idiomatic	3
Makarov	6
Proverb	3
Scientific	1