I love playing my hunter. Pew-pewing mobs while my pet holds them at bay makes me happy. However, I am more nervous playing my hunter in dungeons than I am with any other toon, not because I am unsure of how to play her, but because of other player's attitudes. I fear the dreaded slur "huntard." You see, everyone makes mistakes, especially in the relatively high-pressure environment of a dungeon or raid. But if you are a hunter, you will be held to a higher standard, and any mistake you make will be called out and ridiculed. The minute you zone into an instance, it is likely that other players will be watching you and waiting for a screw-up, which they will make you aware of in the snottiest way possible. For example, I ran Throne of the Four Winds on my priest this weekend, and before we hit even a single mob, the tank asked a hunter if he was "a huntard who is going to pull for me" or something to that effect. So as a hunter, your class alone means that you will be singled out for added scrutiny, and any flaws in your performance noted with disdain and perhaps even a kick.
So, you must make extra certain not to make mistakes when playing your hunter. As the dork tank's comment above reveals, the most typical mistakes involve pulling unwanted mobs, and this most often happens with your pet. Perhaps "huntards" should more accurately be called "pet-tards," as our pets are usually the ones screwing up. So learning how to better control your pet will keep it from embarrassing you in a group.
Before you even get into a dungeon, make sure that you have filled out the talent tree for your pet (press N and select "Pet Talents" tab). This is particularly important if you like to tame new pets like I do, as each one joins your stable with an empty talent tree. Without the points assigned, your pet won't do near the damage it is capable of (BTW, a good place for pet builds would be a website like Warcraft Hunter's Union or something similar).
To make this process a bit easier, I use a macro that I found on WowWiki. The macro casts Hunter's Mark, sends my pet to attack, and begins my autoshot all in one keypress. I have the macro bound to my 1 key as it is always my first move:
#showtooltip /cast Hunter's Mark /cast Auto Shot /petattack
To make the macro, press Esc in game, click the Macro button, and paste this text into the box. You can make an icon for the keybind too; I use the Hunter's Mark icon with the word GO added. After you've made your icon, drag it into your action bar and you're good to go.
The pet stance picture above also indicates with a black arrow the proper aspect for most dungeoning: Aspect of the Hawk. I haven't found much need to use the other aspects with Gallore, so Hawk is always on for her and not a problem.
The last thing I would caution against is not something specific to hunters, but again, since your hunter will be up for special scrutiny, it's good to try to avoid making this mistake. When using an AOE strike against a pack of mobs (Multishot for example), make sure that there are no additional non-aggroed mobs behind them. Hitting a peripheral mob with a Multishot counts as HUNTER PULLS OMG and will get people's panties in a bunch. I actually have this problem much more frequently when using Chain Lightning with my shaman, so I assume any AOE damage ability may occasionally pull unwanted mobs.
That's it for my little hunter guide! Looking back over what I've written, it's clear that the topic gets me a little riled up. This is probably because I have a little hypothesis as to why hunters get so much grief in WoW, mainly:
Whether true or not, the perception seems to be that hunters are played by females, perhaps younger females, and this may be why some players feel that they can get away with abusing them in game. However, there is also a perception that hunters are an easier class to play (I don't agree with that), and so more likely played by new players. Either way, rage-prone players sense weakness in hunters, and subject them to worse treatment than I have ever seen or experienced on any of my other toons. I don't know what the solution to this is, aside from making hunters so OP that everyone is happy to see one join their group...