My understanding is that a "good grade" of tea simply is a tea that
conforms unusually well to tea-ideals of the country that made it. For
instance, Chinese tea drinkers value large, shapely leaves and want
many steeps out of leaves, so expensive Chinese teas are usually
whole-leaf (and sometimes elaborately shaped) and almost always give
multiple steeps, especially oolong.

The Japanese, however, don't value the multiple steeps or leaf style,
so their "high grade" tea doesn't necessarily have these qualities.

That's not even to touch on country-specific preferences in taste
(Chinese earthy vs. Japanese "clean" vs. Vietnamese STRONG). Again,
expensive "high grade" teas will usually be unusually good examples of
the preferences of the the country of origin.

"Leaf grades" are another matter--mostly for Indian teas, I think.

http://www.uptontea.com/shopcart/inf...NFOgrading.asp

Incedently, I find that whole leaves usually give subtler tea, and are
less likely to become bitter. But, chacun a son gout.

ZBL