What makes a game watchable?

What makes a friend or a sibling come down and sit next to you for the sake of watching you play?

That's what I mean by "watchable." It's something that I've been pondering about for quite some time now, but never really got to an answer. None of my ideas have been solidified, or even decently backed up, but I have a good handful of them.

• Personal preference. I, for one, will never find watching a FPS game fun, but I know a lot of people have parties of just playing & watching each other play FPS games. But this really only applies to people who already have a background of gaming.
  
• Familiarity of characters and series. Mario games are so good at filling those couch seats up, and I'm guessing it's a matter of familiarity. Again, just a theory.
• Multiplayer option available. Oo, is this how you play? Can I try? Can I play with you? This is the basis of racing and fighting games.
  
• Theatrical content. Like you're watching a movie! RPGs with a solid storyline do very well in keeping watchers on their seats. Final Fantasies are good examples. Hollywood action-like games are also good!
  
• Puzzles. Make that friend think with you! Like Resident Evil! This may seem surprising at first, but I believe the aspect of Resident Evil that makes it so watchable is that balance of watching theatrical action and being able to understand solve the puzzle / get to the key with the player.
  
• Graphical presentation / Colorfulness. Mario is very colorful. Katamari is very colorful. FFXIII is fantastically colorful. FFXIII is also very breathtakingly beautiful. Presentation plays a huge role in catching that initial glance.

Now, what makes a game unwatchable?

• Nondynamic action. With exception to those hard-core players, games like Starcraft and Dota (Warcraft) are very dull to watch, making glances to the screen last only a few seconds, minutes at most. I say nondynamic in the sense that there lacks a fluidity of camera motion, or theatrical content as mentioned above.
• Confusing games. The system / gameplay is too complicated to understand just from one glimpse. Marvel vs. Capcom 2 comes to mind; characters switching in and out and doing special moves here and there was too much action at once for me to comprehend when I first saw it. It's also what makes the game so great, but confusing for first-timers.
• Size of the screen. This kind of goes without saying; games on portable consoles are hard to watch... You have to sit reaaal close.
• Repetitivity. Games like Cooking Mama, where similar actions are done over and over again, are addicting to play but get old to watch quickly.
  

Again, allow me to mention that these are just unsolidified, random ideas to a question that I randomly began to ponder.