Toughness is a function of fat marbling, one of several important parameters -as pointed out by
@Fervent Freyja
In the US there are meat grades: prime, choice, standard. They are based in part on the amount of marbling. Buying beat quality (grades) of meat improves tenderness.
Taste to a huge degree and tenderness also depends on how the meat has been handled, or "aged". High end restaurants keep large sections of green beef hung in a cooler to dry age for a minimum of 30 days. Periodically, the outer dry layer that develops is shaved off with a special tool to allow moisture to continue to leave the meat. There is also enzymatic activity during the dry ageing beef which alters taste and tenderness a lot. A 20 pound cut of beef can lose 40% of it's wet weight during this process.
...why dry aged is expensive.
Cryopac (wet) meats have been dry aged for a few days then wet aged: both for much shorter times and consequently are less expensive. Plus they have not lost as much moisture so are cheaper to produce.
Some meat places have dry aged beef which is often much more expensive that the wet pack. You can dry age at home.
Buy a prime grade roast at the grocery store, they are easiest to dry age a few extra days at home:
http://www.finecooking.com/articles/dry-aging-beef-pays-off-big-flavor.aspx