It's interesting for me that the Pokemon Mystery Dungeon series brought me more enjoyment than most other main series titles by a longshot. An emotionally charged plot, even unchanged game from game, is the foundation for what made PMD games so special to this day. From game to game, we know exactly what's going to happen because like main series games, the plot doesn't necessarily change, but the action leading up to the climax does, and that's the most important part of the PMD games for me! To see the story unfold, to see my Pokemon and their partner journey throughout many towns and go through triumps and suffering to face the Big Bad of the games at the end of the day... it has much more depth on an emotional level, to me.
For anyone who hasn't played the more recent PMD games yet (except Gates to Infinity, that was awful), you are really missing out on some great games. Highly recommend Explorers of Sky, although depending on your local game retailer, it might be hard to find and quite expensive because of that. People priase it as the best PMD game to date, although I'd argue Super Mystery Dungeon is pretty much on par.