Tea comes from a plant that, if you look at most plantation photos, seems to grow about knee high or less. However, if you look at photos from China, where the plant has grown up, it's a 25-30' tree. It's exactly the same plant, but some places keep it small by picking a lot more of the leaves, others by not having quite the right soil or climate to allow it to grow. The height, age, climate and soil all affect the taste you get - although I can find very little information on how. Most biochemistry research on tea assumes all black teas are the same and all green teas are the same (and that those are the only two sorts). As none of that is true, and as falsifying the axioms is a valid way to falsify the conclusion, I therefore conclude that most research on tea is bunk.
The tea can be picked at different times of the year and, again, this appears to alter the taste substantially. Which is what you'd expect. Not only is the climate different at different times of the year, but the activity of the tree is different, the level of photosynthesis will vary and the soil/root bacteria (which actually feed the tree) will have altered the chemistry around them.
Next, there's the processing of the tea. This involves some combination of fermentation and oxidation, though different methods will use different ratios and probably different reagents. Different recipes also use different parts of the leaves. (It's hard to tell on the specific details, it's not like the tea gardens publish their secret recipes.)
Finally, there's the aging, which can be anywhere from essentially nothing to twenty or more years. This is a mix of breakdown of material by bacteria/fungi, natural breakdown of unstable molecules, slow reactions between chemicals present and probably a bunch of stuff that shouldn't be read on a weak stomach. Hey, it's well-known that that's a technique used in making coffee!
Here, I throw it open to the tea aficionados. If a tea genie popped out of the kettle and offered you a crate of any tea you liked, and even let you produce your own blend of teas, what would you actually go for? What factors actually matter, in the end?