Criticizing the speaker or the delivery is one thing. I think that bad listeners often miss the message because they are too focused on the speaker or how the message is delieverd. Bad listeners tend to miss what is being said because of who is saying it.
Eye contact was already mentioned so I would kind of like to add on to that faking attention. I know a lot of people who can fake listening and make it look like they are paying attention but really they are just daydreaming. Body language and eye contact are important but they can easily be faked.
I think sometimes it is letting emotions block the subject. We all have biases when listening to someone talk about a subject, but I think good listening involves listening without a bias. Bad listening tends to ignore what the speaker is saying because of previously held ideas and opinions about the subject.
A big one that annoys me is rushing the speaker and making him feel that he's wasting the listener's time. It annoys me to see people who look at their watch or flip open their cell phone during a conversation. I have a friend who has a nervous tick about checking his phone all the time and it just always makes people feel like they are unimportant or they are imposing on his time.
Another big one is trying to finishing or complete the speakers thoughts. I think that shows a lack of respect for the speaker and really shows you aren't listening.
The biggest one for me is when people try to out do the speaker or one up them. It kind of shows a disrespect for the speaker by saying that your experience is more important or more valid.
I think for me bad listening is best summarized as a lack of respect for the speaker. Whether it is time, interupting, not giving eye contact, bad body language or whatever else I think it all boils down to a lack of respect for the person speaking. It is probably not in a conscious thing most of the time, but it still shows a lack of respect and devaluing the conversation.