Water far too hot
The most common by a wide margin. Boiling water on a delicate green or white tea extracts the harsh compounds efficiently and the sweet ones poorly, producing a cup that is bitter and thin at the same time. If tea has put you off, this is the first thing to change: let boiled water stand a few minutes, or add a splash of cold.
An infuser the leaves cannot expand in
Dry leaf swells to several times its volume. In a mesh ball or a clip spoon it cannot, so water only reaches the outside of the clump and the tea brews weak. The usual response is to add more leaf or steep longer, which produces something weak and harsh together. A wide basket or a strainer on the way out fixes it outright.
Guessing the quantity from a teaspoon
A teaspoon measures volume and teas of equal weight occupy wildly different volumes — a spoon of rolled oolong can be six times the weight of a spoon of fluffy white tea. One instruction cannot serve both, which is why the same advice produces good tea from one packet and undrinkable tea from another.
Leaving the leaves in
Tea left sitting in water keeps extracting, so the last mouthful is not the drink you started. It also ruins the next infusion before it begins. Pour off completely, every time, and re-steep deliberately rather than continuously.
Judging a tea on one steep
Many whole-leaf teas are better on the second or third infusion, when the leaf is fully open. A tea assessed on a single Western steep has been assessed before it started, and this is especially true of rolled oolongs and compressed dark teas.
Old tea, and not knowing it
Tea does not spoil but it fades, and green tea fades fast. A flat, characterless cup that brews to the right strength and colour is usually a storage and freshness problem rather than a brewing one — a different fault entirely from weak tea, and one no technique corrects.