When I've sang "I like big buds and I cannot lie" for several days, it's a sign that I need to write about this subject.. The main message is: be on the lookout for presence of buds in your tea! If there are none, it means the leaves that have been plucked are of inferior, older quality. You can recognize the buds from the wiry form and the small white hair that's on the bud protecting the fragile leaf before it's time to open up to the world. In black tea the buds are golden, because of the oxidation processing.
Real quality tea always has buds in it. Why? Because the unopened leaf which is still in the shape of a bud is the newest part of the tea plant, containing the most flavors and nutrients (and caffeine!). The bud is the smallest part in the plant, followed by the next two leaves. Most often in high quality teas the bud and the upper leaves are adjacent, to prove that the two leaves below the bud really are the second and third leaves, not the older and bigger leaves. There is a term in the tea world describing the picking of this part: "two leaves and a bud" and then you know the best parts are beings plucked. This is because these parts have the least surface for the aromas to disappear to thin air, they're the freshest parts and they soak up all the sunshine.
Because buds and the following leaves are so small, it takes so much more effort to pick just that part in comparison with bigger and heavier leaves that grow lower in the plant. And the crops is of course smaller and more expensive, because of the thorough time-consuming picking. And these parts you really want to nurture with love and care to create the best teas, so the processing is always slower.
The leaves that grow lower in the plant have been aged for a longer time, growing their surface and from that big surface the flavors and nutrients disappear quite fast. These lower quality leaves are used to create tea for cheap mass-production. There are a couple of exceptions, though: oolongs and puers are often made so that the older leaves are picked as well, to endure the rolling and processing.