Properties of a good steak?
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.