The thing that I think makes Tetris so great is its simplicity. I often measure the greatness of a game's design by postulating whether or not it could have existed on much earlier hardware.
Thus, Tetris could have run on an Atari 2600 because it's not a graphically intense game, and yet it is gameplay intense. Similar phenomenon can be seen with Dance Dance Revolution (Foot pads and music existed before the CD console era), Dwarf Fortress (ok, well, maybe not. It can choke a modern CPU), Angry Birds, Minecraft, Peggle, and all the MOBA's (which evolved from 10 years of older games that evolved this modern style). The soul of good game design is constrained resources.