It may seem simplistic, but I believe great acting a major component in movies that make you say, "Wow."
There are a few things that make a performance great acting.
Do I believe in the character? In other words, when the actor is acting sad, do I believe the character is actually feeling that way? Or do I think to myself, "Boy, this guy is acting sad in order to earn his huge paycheck"? See: Keanu Reeves.
Is the character being portrayed consistently? A lot of this depends on the writing, but do I get to know the character so well that I can easily predict how he or she will react in any situation?
A simple example here is Bruce Willis in the Die Hard series. When the bad guys kidnapped McClane's daughter in Live Free or Die Hard, everyone knew he'd hunt them down and kill them. On the other hand, if I'm constantly scratching my head over how the character acts, I chalk it up to either bad acting or the character is bi-polar. The last factor only applies to well-known actors like Julia Roberts, George Clooney, Brad Pitt, Sylvester Stallone, Renee Zellweger, Matt Damon, Natalie Portman. Christian Bale, Michael Caine, etc. Often when I'm watching movies with these actors in them I find myself thinking, "Well that was a dumb thing to do, Cameron Diaz," because she's playing herself.
Will Smith is often in this category, but in I Am Legend, he did a good job of playing someone other than his wise-cracking, super-cool self. As his character Neville desperately tries to maintain his sanity while deprived of human company, he slowly slipped into instability.
Bottom line, when an actor can make me forget that I recognize him or her, I'm impressed. Examples include Russel Crowe (A Beautiful Mind, Gladiator, Master and Commander), Christian Bale (Batman Begins, The Prestige), Cate Blanchett (Lord of the Rings, Elizabeth).
So there you have it. Some actors can pull off legendary performances on a consistent basis, while others are somewhat hit-and-miss. Jim Carrey often does his slapstick-manic character, but sometimes he'll do a Man on the Moon or Truman Show and blow everyone away. I suppose this is because really becoming a character is a difficult task.
But that's the way life is. We appreciate it when people make an effort. With actors, their work is so much more visible than the average Joe that they come under scrutiny a lot more.
I'm a graduate from BYU-Idaho with a degree in Communications with a print journalism emphasis. I currently work as a test engineer for a software company. I've been married for seven years and have three kids.