As an avid gamer I've always wondered what small things made one game particularly outstanding next to another. One game may be more "pretty" to be sure and gameplay mechanics are always important. But sometimes one game just seems to get us to play more than the others despite being in the backdrop. Nostalgia aside, I think the early games (like Solitaire) did (and do) just that. Nintendo is certainly cleaning up the floor with this idea of a game that you don't want to put down (despite that pain in your shoulder).

More germane to the topic is trying to flesh out what makes solitaire invoke that sort of "one more round" feeling you get when you are done. Whether it be Vegas or just regular. Some games capture this such as the highly acclaimed Civilization line of games. Also Simcity is a good one for "just one more year to see how things change".

In conclusion, I think all gaming studios are reaching for that one subconscious reason we continue to play minesweeper/solitaire/etc. Just like companies are looking for that jingle that no one can forget (I hate you Mentos) I think getting down to what causes us to play over and over again is vital to an expanding industry. What makes us pick up our controller/keyboard/mouse and go back through something time and time again?