Dad’s short answer: Five, in order: extraction strength, a proper pet tool, a dry or suction-only mode, tanks that are easy to fill and rinse, and formula availability. Everything else — brush count, tank size claims, heat — is a tiebreaker.

The five that matter

Extraction strength first, because it's the mechanism. A machine that sprays well and extracts poorly leaves your carpet wet and the stain driven deeper. Judge it by whether the dirty tank fills.

A proper pet tool second, if you have animals. The ones that trap solids in their own filter instead of pulling them through the hose turn an awful job into a rinse-the-tool job.

A dry or suction-only mode third. Being able to make passes with the solution off is what gets your carpet dry in hours instead of overnight, and it's the single most useful control on the machine.

Tanks that fill easily and come apart for rinsing fourth. You'll do this after every job, and awkward tanks are how machines end up unrinsed and smelling.

Formula availability fifth — check before you buy whether the solution the manual specifies is something you can actually get.

What you can ignore

Brush count. Twelve brushes isn't better than four if the four are well designed and driven properly.

Advertised tank capacity on its own, since the dirty tank is what limits you and it's usually the smaller number.

Heat, unless you're dealing with grease or set-in stains regularly. Useful, not decisive, and often warmer rather than hot.

Anything described as steam on a machine that's really heating water.

And weight quoted without context. Balance and wheel design decide how a machine feels far more than the number on the box.

Dad’s Take: Extraction, pet tool, dry mode, easy tanks, formula you can buy. Get those five and the rest is decoration.