I'll provide a more indepth opinion, considering the fact it's really more of an opinion based topic.
At it's core, I guess a good gig poster is one that someone buys, or makes someone want to see a show, but that's common sense.
You need to decide on the purpose of the poster. Is it to advertise a coming gig, or is it a souvinere of the gig? Maybe it's got nothing to do with either and is simply to advertise the band as merch. Souvinere posters are going to be more visually creative and "artsy" where as a promotional gig poster for a coming gig is probably going to be a little more corporate and informative. Mind you I'm noticing a good blend with advertising gig posters being quite creative in themselves.
To me, a good gig poster communicates. It obviously has to fit the theme of the band. The typography has to be fitting, creative and artistic but not too cluttered. You need to consider the colour of your poster. You want it to grab attention away from all the other gig posters that will be on a wall at any time, so you may need to consider the colour of the paper, or the colours of the ink you'll be using. Colour use will be important when linked with the theme of the band, a bight colourful poster may not be "good" for a metal or rock band, where as dark colours and blacks won't necessarily be fitting for a pop artist, or a musician with upbeat music... these aren't rules but more considerations.
Then, depending on the purpose of the poster, if it's an advertising poster, it needs to have a good blend of information and creative art to draw you into seeing the show. Maybe it needs to be a concept driving visual with information mixed in, or it's almost purely information based with little illustration. A souviner poster isn't going to be about promoting the show, the buyer has already seen the show, they want to be reminded of the show and why they love the band. Those posters are more likely to be highly conceptual and visual and again, what's "good" is up to everyone else's opinion. I've seen some fantastic posters, and then I've seen some messy looking posters that look a little too thrown together. To other people, they may like the thrown together aspect.
I see an assignment like this as being more about asking people's opinions on the subject seems it's a hightly subjective topic. What's "good" really is up to the person.
And maybe instead of what makes a "good" poster, perhaps ask, what makes an EFFECTIVE poster?