Bad agents:
1. Send someone else in their place that doesn't know anything about the house. I understand that some heavy listers can't be everywhere at once, so they enlist aid, but some don't bother to make sure the stand-in knows anything. Here are some actual exchanges:
Me: Do you want some feedback?
Agent: (shrugs) It's not my listing. It's Janice's.
Me: Do you think she wants feedback?
Agent: I guess you could tell me if you want.

Me: (shaking head incredulously) How did she (the listing agent) come up with the price?
Agent: I don't know, maybe the seller insisted. It's probably about $100,000 too high.

If you're not making the effort to present the house well for someone else, you probably won't for me either.

2. Doesn't make sure the house looks good for open house. Yes, I realize it's technically the owner's responsibility, but you need to be a better coach. I've seen vacant houses with weeds in the beds, cobwebs on the windows and trim, and without heat or air turned on sufficiently in advance to make the house comfortable for showing.

If you ignore this stuff, you either don't have the attention to detail that will market my house in the best light, or you're lazy.

3. Leave before the open house is over. Yes, I've showed up at open houses 15 minutes before the end and sign is gone, door is locked.

Someone who comes late and leaves early...not who I want working for me.

4. Watch TV, talk on the phone, or text while people are walking through. No, you should be selling.

OK, I'll balance this out with good ones.
1. Talk to each person that comes through. Introduce the house up front BRIEFLY and ask them for feedback when they leave, even if they're not a potential buyer. Here's a good exchange.
Me: Do you want some feedback?
Agent: Yes, please.
Me: It's got a gorgeous view, but with the work it needs to update it, it's significantly overpriced.
Agent: Thank you. I'll make sure to pass this along to the owner.
Note that he didn't agree with me or down-talk the house, though I'm sure he was biting his tongue.

2. Have lots of info about the house to answer questions. One agent I saw had a map of the land showing proximity to neighboring houses. Our agents had before and after photos of our house to show buyers who said "this was built in 1994?" so they could talk up all the upgrades.

3. Stay late. Our agents had an appointment across town right after our open house, but stayed another 15 minutes because a buyer was lingering and asking questions.

Any other insights or experience to add?

I also hate agents that are all about appearances and don't know or care about things like easements or drainage. I hate the ones who say "I'm pretty sure you could have horses here." I'd have to be really stupid to rely on that, and it just makes everything else they said suspect.

I wish I could say I had an agent that found me a great deal or helped me word the contract to my advantage, but not yet.

A good agent knows the internet and knows how and where to promote your home; a bad agent just puts the MLS listing up and figures he's done his part. One of ours had our house on MLS, on Craig's list, had targetted ads on Facebook aimed toward the kind of people who were most likely to be interested in our home and neighborhood.

A good agent gives you help knowing what things will appeal to buyers--a bad agent will come in and tell you how nice your house is.

Guess which of our 2 agents sold our house? in 6 weeks? a direct result of the second open house he held for us.