What are the key components that make casual games fun?

This is something that I'm going to have to more seriously investigate. Normally I like a balance of solid core gameplay (the mechanics are layered on in a fun and appropriately challenging and rewarding way) and great production values (ideally in the form of great story, polished sound and visuals, non-threatening UI, and few noticeable bugs).

That said, all these components are really just slider bars. Shoddy story can be outweighed by great gameplay. A beautiful world can make me want to explore even if controls are lacking. Bugs can be overlooked when they bend but don't break my immersion.

What I'm really trying to figure out is why I'm playing so much Puzzle Quest these days. In some ways I know I'm just filling the gaps while I wait for some other games to arrive (mainly Mass Effect and Uncharted: Drake's Fortune. But, still, I have Blue Dragon, Assassin's Creed (which I'm determined to play for another 3-4 hours before making a final decision) and some new demos to try (including Culdcept Saga, a CCRPG of some sort that sounds kind of interesting).

It could also be that I just haven't had big blocks of time lately. My birthday weekend was comprised of lots of commitments (which were fun) and didn't really provide any large gaps in my schedule that are required when starting a new AAA game. Sadly, things aren't going to get much better over the next couple of weeks -- the holiday season seems to conspire against big uninterrupted blocks of time.

I think in the case of Puzzle Quest, it comes down to a few basic things:

• The core mechanic (matching jewels) is rewarding on multiple levels.
• The puzzles have been well balanced to keep things tense (risk of failure) but not frustrating (at least on default difficulty). I generally win against opponents I should beat, I have close matches against equal or slightly more powerful enemies, and I get destroyed by much more powerful enemies -- which inspires me to got out and rack and acquire new skills and figure out new strategies.
• Solving puzzles makes me feel smart and strategic. Yes, the main task (matching jewels) is repetitive to the extreme. But you learn quickly that you need to be creative (and not just reliant on one strategy) in order to progress through the game.
  
• The game makes good use of visuals, sound, and vibration so that the actual swapping of items and casting of spells is viscerally fun. I can't imagine, for instance, that this game would be as fun on the PS3 without the weighty controller vibration when a row thunks into place.
• The character progression system is very rewarding. In addition to leveling the character, there are spells that can be researched, items that can be formed, mounts that can be trained, items that can be bought, etc. There are all sorts of little economies going on which generally means you are always "just one more game..." away from leveling up or acquiring something cool and new. This definitely adds to the addictive nature of the game.

All that said, my goal for the next time I have a small block of time: Try the Culdcept Saga demo.