Of course, there's nothing inherrently wrong with random groups, especially if it's the only way you'll get to run that instance. But the bad reputation stems from the fact that a lot of people really don't know how to behave properly. The phrase 'does not play nicely with others' comes to mind.
So this is my guide to playing nicely. And I'm going to ignore the fact that people who read blogs are probably not the target audience for such advice. I need a post and this is it, so there.
Be prepared. If you come to an instance without food and water I will quit. Even if you're expecting a mage. Even if it's only pet food. Don't forget your regents. That means you too shaman. Remember those ankh things? If your armor goes red after the first wipe, I won't be hanging around for you to repair and return. If you whisper that other hunter in the group and ask for ammo, don't be surprised if they kick you. Be prepared. It only takes five minutes, it makes you look professional, and it shows your respect for the other players who have all taken the time and money to prepare too.
Know your class. And know everyone else's. Every class has a bunch of abilities that they just don't need for levelling. But when you walk into an instance they suddently become rather handy. Dropping the right totem or popping the right aura will mark you out to the rest of your group as someone who knows what they're doing. Not everyone will be familiar with your enounter though, so it's useful to know what other parties can bring to the table. Could we have aspect of the wild please mister hunter? Would you mind using curse of recklessness here please missues warlock?
Be polite. Say please and thank you. This may not be the real world but these are still real people who deserve respect. You'll be amazed how a few nice words can change the mood of a party and even improve performance. And if things go wrong don't shout and swear and stamp your feet and blame other people - even if it is their fault. What you should do is 'fess up when you cock up. Everyone makes mistakes and it's no crime to admit to them. Just say you're sorry and move on.
And be considerate. This means waiting for your mage to drink, letting your tank set up the pull (unless you've agreed an alternative strategy), asking before skinning, herbing, minig, and waiting patiently if a player has to go afk a sec.
Play for the group, not for yourself. This one is more for dps than tanks or healers. Make sure your objective is to beat the encounter, not just top the damage meters. Don't push your tank's threat, and don't waste mana on inefficient actions (warlock curses, for example: agony does damage but elements may be more beneficial with a mage in your group).
Be fair with loot. Yes, I know, this is probably the only reason you're in the group in the first place. But if you're an elemental shaman don't roll need on some hunter mail cos you'll just get a bad rep and nobody will ever play with you again. It's fair to ask if you can roll for off-spec gear if nobody else wants it. Chances are there will already be unwritten loot rules on your server but if in doubt ask the rest of the group before rolling.
Don't post damage stats. Ever. Anyone who cares about damage enough to want a printout will have their own meter. They are only useful for measuring personal performance anyway. And the total quantity of damage done rarely reflects a player's contribution to a group. Plus, only wankers post damage stats.
I'm feeling quite smug at this stage as I've just realised these are all the things that my guild do anyway. Do you think it's asking too much to expect this behaviour in a pick-up-group?