The key to make a game appeal to both types of gamers is to include elements such as 'rubberband AI' (In Mario Kart, CPU speed was adjusted by player position; items get better as you fall behind) and difficulty levels (imagine that!), choices, or chaos (The Super Smash Bros series can turn a near-win into a loss or a tie through items or stage elements). Super Mario Galaxy 2 allows players to choose between easy, doable and brutally difficult Stars. Frequent checkpoints and lives ensure that the challenge is painless for those who wish to undertake it. Since you only need 60 or so to win, the challenge is self-adjusted. Unfortunately, as games have become more complex, tweaking hardness is nigh impossible without losing balance or frustrating players; this is a brilliant little strategy that leaves all players fulfilled.
Now, 'casual' games made in 5 minutes are a different story....