Properties of a good steak? I'm talking about what makes a good piece of meat for a steak.

not how to cook it.

What type of cut? Thickness? dry aged? to marinate or not? seasoning?

I've always been partial to a Ribeye (high fat content), cut to a minimum of 1.5 inches to achieve a nice crispy (more brown then grey) outside with a red interior.

lots of pepper, little salt.
I've come across two great, and very different, types of steak. The first is typical in Argentina and is not aged at all, but from a young calf (6-10 months old), free ranging and grass fed (hence not like classic veal). This is really juicy, tender, and with a very "bright" taste. Good high-end Argentine restaurants usually serve meat like this, and in the UK you can order it from pampasplains.com, which I think is great.

The second is from an older cow and dry aged to tenderize, but the aging also gives it a deeper, more bone marrow-like flavour. The best I've had of this type was a cow from Limousin in France, served in Paris at a small restaurant called Le Sèvero (8 rue des Plantes, 14th Arrondissment, reviewed in the NY Times a while ago). It was amazing! In London, 32 Great Queen St's "Hereford beef" is a similar style and also excellent. They get it from a Hereford farmer called Tom Jones (seriously), who serves several restaurants.

Good steak should never be marinated in anything - all it needs before cooking is salt, maybe pepper if you like (though I sometimes eat it with Coleman's English mustard).

I'm fairly sure that grass-only feed is important to good meat, i.e. no cereal-based fattening-up, though it's hard to separately test this one variable. Breed also clearly matters (all Argentine beef seems to be Aberdeen Angus or Hereford). I love a nicely marbled, fatty steak like the ribeye, but my (Argentine) wife prefers leaner cuts like sirloin and filet - clearly a matter of taste.