I'm a theatre director, and I'll list the traits I think of as bad, though "bad acting" is subjective. I was reminded of that today, when my wife, Lisa, and I finished watching a show. Lisa has as much experience in the theatre as I do; she's very smart and has excellent taste. But, after the show, when I was done railing about how bad the lead actor was, she said, "I thought he was really good." So take my opinion (or anyone's) with the grain of salt it deserves. (Though, of course, I was right and Lisa was wrong.)
1. Emotional armor. When I watch actors, I want to see vulnerability. One skill great actors have is allowing themselves to be (emotionally) naked in front of a lot of strangers. This is extremely hard, as we spend our lives learning how to not do that. I am not necessarily talking about wailing and crying. Watch Anthony Hopkins in "Remains of the Day" and you'll know what I'm talking about.
When I work with actors, the biggest hurdles come when they try to protect themselves. Different actors do that in different ways: some flinch from the uglier sides of human nature*; others have certain feelings they'd rather not explore; some simply don't want to look unattractive. One of the many reasons Bryan Cranston is so great (see "Breaking Bad") is because he'll do scenes in his underpants -- literally and figuratively.
* I once worked with a good actor who had a serious emotional wall. In real life, he was a true gentleman: the kind of guy who would run into a burning building to save a child. But in the play we were doing, his character had to watch while another man molested his girlfriend.
The actor had immense trouble with the scene (and I don't blame him). He was supposed to stand there and do nothing. He kept coming to me and saying things like, "What if I try to fight, but some of the bad guy's henchmen hold me down?" I couldn't let him do that. The point of the scene was that his character was totally cowed by the bad guy. His character simply loses his nerve -- becomes unmanned. The actor didn't want to explore being unmanned, and he didn't want the audience to see him that way. It was a problem!
2. Pushing. Stanislavsky, the great Russian acting theorist, helped us understand that acting works better when actors pursue goals rather than try to emote.
Bad actors think their job is to emote. And when they're "not feeling it," they force it. See Mel ("Give me back my son!") Gibson and Nicholas Cage.
3. Lack of confidence. It simply takes time to feel comfortable on stage or in front of a camera. There are many actors who will be wonderful some day, if they stick with the craft, but they simply don't have confidence yet. The stage makes them nervous, which causes them to hold back. It cases them to protect themselves. (See point #1.)
I've lived with an actress for almost 20 years now. We met in grad school, so I saw her act when she was first starting. She was obviously talented then, as she is now, but it's been fascinating to watch her confidence grow. There was a time when she couldn't have carried a show -- she couldn't have played Lady Macbeth or Kate (the Shrew in "Taming of the Shrew") because, even though she was talented, she would have thought, "Who am I kidding?" Now, she commands roles like Beatrice and Clytemnestra, and the stage is her home. (When actors start out, the stage is someone else's home and they're visiting.)
Of course, even seasoned actors have moments of doubt, but they get to a point, as most people do in most professions, when they feel like "I actually know what I'm doing, I enjoy doing it, and I can do it any time, anywhere, under any conditions, and I can recover from mistakes." A young actor -- even a very talented one -- will fall apart if her costume gets torn or she forgets a line. A seasoned one will take it in her stride, and the audience can feel that confidence, even when nothing goes wrong.
Putting this together with my first point, an actor must be confidently vulnerable. That is a tough thing to be!
4. Discomfort with language. This is easy for me to spot, because I mostly direct Shakespeare, but it's an issue no matter who wrote the script, because words are the actor's main tools. Some people are comfortable with words: they know how to use them as weapons and aphrodisiacs. Some people love feeling words in their mouths, rolling off their tongues, broadcasting outward. Others trip over them.
5. Discomfort with their bodies. Actors can be of any physical type. There are great skinny actors, great fat actors, great tall actors, great short actors, great beautiful actors and great ugly actors. But however they look, they need to own it and know how to work it. (See John Goodman.)
6. Untrained voices. Humans have a huge vocal range, but most people don't use theirs. Since an actor's main tool is speech, he must know how to, in Shakespeare's words, "Speak the speech ... trippingly on the tongue." (Please get some vocal training, Keanu!)
7. Over-trained voices. On the other hand, you can go too far, training your voice into something that sounds unnatural and actorly, as William Shatner did when he was younger. And thank goodness Vincent Price mostly worked on schlocky horror films. If you want to hear two beautiful (for normal-sounding) voices, see Henry Fonda and Tony Curtis in "The Boston Strangler" or Kenneth Branagh in anything.
8. Pre-planning. Good actors spend a lot of time thinking about motivation, movement, ways to say lines ... and then they let it all go and just see what happens. The point of preparation is to make your mind as ready as possible to be unleashed. But if you don't unleash it, nothing surprising or dangerous can happen. Bad actors don't understand that planning is the groundwork for inspiration. They stick to the plan, no matter what happens. This is yet another way to escape from vulnerability.
9. Bad actors don't listen. They deliver a line, pause for the other actor to speak, and then deliver their next line. When they're not "on," they're off. Good actors know that speaking is just 50% of the job. Listening is equally important. Notice how often, in a film, the shot is the listener, not the speaker. Notice how often, when you see live theatre, your eye is drawn to the actor receiving information. You can't easily fake listening. You have to really do it. It's dangerous, because if you listen, and the actor you're listening to speaks in a way he's never spoken before, you may have to stray from your plan when you respond. See point #8.
10. Bad actors don't warm up. This only applies to the stage, but it's crucial. When actors don't spend ten minutes (or however long they need) vocally and physically warming up, they are not ready to start when the show starts. And then what happens is that the first ten minutes become the warmup. Have you ever seen a play that was good -- but it didn't really click until ten or twenty minutes in? That's often because the cast didn't warm up before. Instead, they did it on your time (and on your dime).
In acting school, actors learn to warm up. Too many of them quit doing it once they graduate, hubristically saying, "I don't need to warm up." They think of warmups as schoolish and childish. They think they can get away with hanging out in their dressing rooms, chatting and eating pizza -- and then strolling onto the stage. They can't.
Contrast them with Derek Jacobi, who used to speak the entire text of "King Lear" -- well, his part in it, anyway (he was playing Lear) -- before the show started. By the time the audience arrived, he was "in the zone."
11. Finally, good actors are smart, have great taste, love collaborating, are extremely hard-working, are humble (not divas), are generous, are inventive, are playful, are adventurous, have sense of timing, and a sense of humor. Bad actors are lacking one or more of those traits.