What makes a great story?

I gave myself a little homework assignment over the holidays. I tried to find out what makes a short speculative fiction story great. I probably read over a hundred stories from those "Best of" anthologies. What did I conclude? Every great story is different, there are no hard and fast rules. Okay, in hindsight, this may be obvious. :) Another obvious thing: opinions about stories are subjective.

Here's what I took away from the exercise...

• an excellent story engages the reader's emotions. Note, too, serious emotions such as poignancy, sorrow, or joy are more effective than humor.
  
• an excellent story really tortures (figuratively, not necessarily literally) the protagonist. Often the protagonist is an underdog, starting the story behind the 8-ball (or whatever metaphor you prefer).
  
• many great stories have empathetic, and even sympathetic, protagonists. Interestingly, this is one of Donald Maass' instructions in Writing the Breakout Novel.
  
• many very good stories have unique voices
  
• most excellent spec fic stories have fully fleshed-out worlds. The reader feels like they are actually in another place and/or time
  
• most excellent stories follow the standard plot arc: the protag has a problem and acts to solve it
  
• most great/excellent stories are about more than one thing and these things are interrelated. Often this involves an external and internal plot arc.
  
• most great stories involve some kind of originality--a new twist on an old idea, or ideally, a totally new idea

I will continue to study the issue...

Anyone out there have any insights? What do you think makes a story great?