Part of the taste of the tea will be from the minerals dissolved in the water; but bear in mind that different brands of tea also have different tastes. So if you drink PG Tips in one place and Typhoo in another, you can expect a different taste (even with the same water).
Too much scale in a kettle makes it less efficient (and hence boiling the water takes more energy) as the coating of scale on the element makes transmitting the heat from the element to the water take longer.
I live in an area where the water is hard enough to cause scale to deposit in the kettle, and I descale it every 3 months or so using a product from a supermarket, being careful to follow the instructions.
I'm not descaling because of health concerns or anything like that -- I just don't like getting flakes of limescale at the bottom of my cup of coffee!