When players encounter one another in the wild while questing in the open world, it's a nice touch to take the time and say hello. It's not a big deal, and only rarely will folks castigate one another for not giving a polite greeting -- but it's still a nice touch.
Even more importantly, if you join a dungeon group, raid, or battleground, definitely take the time to say hi to your group. There is very little more irksome than going through the queue and grouping up with four other people, only to be greeted with stony silence. It makes you feel like you just farted in the choir pew. A simple "howdy!" can make the experience much more relaxed.
This is a much bigger deal when you're questing in the wild. As you kill mobs toward your kill count or for a necessary quest item, you often encounter other players who are working on the same quest. It can become obvious very quickly that both you and the other player are using the same resource.
You have a couple of options here. You can say hello and then offer to share the mobs without grouping. That gets a little complex, though, because it's hard to effectively rotate who's killing which mobs.
The better choice is to offer to group up with the other player. The game has built-in systems that give you both credit for kills and allow you to rotate who gets the important quest drops. It only takes a few moments to ask and start the group, and it's usually the polite thing to do.
Roll need, but offer to share
Most heroic dungeons are fairly quick and effective. It doesn't take long to get a basic set of gear. But the accepted rule of thumb is that you should only roll need on the gear that's appropriate to role you used to join the dungeon. For example, if you're in a dungeon as a healer, you shouldn't assume you can roll need on the DPS gear.
That doesn't mean you can't possibly win it, though. Simply ask the group if it's okay to roll for off-spec gear. Heroic gear is awesome, but it's not the bleeding edge of content. Many players have moved beyond needing it, and they'll usually be just fine with your rolling.
The important part is that you need to ask first and not make the assumption.
Don't make assumptions
If you've not picked up on the pattern, the important factor in all of these guidelines is to avoid making assumptions. Take the time to talk to your groupmates, acquaintances, and other people. A few seconds of conversation can go a very long way to avoiding trouble and being more polite.
Read more about WoW's social conventions and netiquette in the WoW Rookie Guide, and get help unravelling the most knotty situations with advice from WoW Insiders Drama Mamas.
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to pulling together enough cash for mid-level expenses such as mounts, to dungeoneering and travel tips for lowbies.