Tetris is a classic.  The core loop is almost as simple as it is unintuitive.  All of the goals, rules and controlls are likely to be discovered within the first play session, and they never change.

One thing I like about tetris is that it gets harder as you go along.  This is common in theory but rare in practice.  Take the counter-example of an MMO:  it seems like PvP combat "gets harder" a opponents get better skills and more knoledge and complex timing is required, but measured agains the same yard stick it does not.  If a lvl 80 character in WoW attacks the lvl 1 pig in the starting zone, they anihilate it instantly.  It has in fact gotten easier for that player to play the game.  In tetris this is not so!  In tetris clearing the first line is much much easier than clearing the 1000th line.

Players are more likely to play a game where they are able to win at first, because it gives them at least some measure of positive feedback.  That way they can optimize on their positive feedback in further play.