Four squares at a time, falling. The whole design is about fairness: a bag of seven pieces, so the worst drought is thirteen pieces, and the piece you need is never further away than that.
With a purely random generator you can be given twelve I-pieces in a row, and there is nothing you can do about it. The seven-bag system deals from a shuffled bag of one of each tetromino and only refills when the bag is empty, so the longest drought is thirteen pieces and the mean is seven.
Everything else in a good implementation exists to make the same promise. Wall kicks let a piece rotate in a tight space instead of locking up. Ghost shows where the piece will land. Lock delay gives you a moment to move a piece after it lands, which is what stops a fast player from losing a rotation they had earned.
Clearing one line is worth 100, two is 300, three is 500 and four is 800 — the jump from three to four is the whole point, and it is the only moment in the game that feels like winning rather than tidying. Lines raise the level, and the level sets the speed, so the scoring system and the difficulty system are the same number.
A soft drop earns one point per cell and a hard drop earns two per cell, which is enough to make dropping a piece quickly worth a little and not worth wrecking your board for.