Taking your low level friends through an instanced dungeon with your high level character is just about the worst thing you can do to "help" them! Its generally quick and easy and they get good gear from it as well as reasonable exp but is still evil.
Catching a ride on your coat-tails means that the newbie is getting good gear without having to work for it. This spoils their sense of achievement, not only for the dungeon concerned, but their questing/levelling until their gear again falls behind. Levelling becomes too easy and becomes boring quickly.
It teaches them nothing about how to play their character. Most runthrus just involve the newbie looting the corpses of the slain - there's nothing educational in that! Lazy, ignorant newbies get themselves into higher level dungeons with no idea how group play works. They become a liability that no-one wants to group with.
It builds the expectation that you will always be available to help them out. More runthrus, free stuff, mount money... the expectation never ends. Giving in to any of this just feeds more begging behavior. Stop the rot at the start and don't do it!
All this of course mostly applies to someone's first character. Runthrus for alt characters are less evil though some of the problems remain. Of course its almost necessary for Twinks to receive runthrus - but twinks are their own brand of evil anyway.
What sort of things can you do to help newbies without spoiling them? Well, I suggest doing things that remove some of the frustrations of low level play.
Give your newbie a set of 8 or 10- slot bags. Bag space limits really suck. Dont give out higher than 12-slot bags though, thats making things too easy.
Clothies could be given a wand. Priests especially suffer from crap DPS in the early levels. A wand, plus instructions on how to use it, are a great help.
Its generally not worthwhile giving gear or enchants to those below level 20, since they will outgrow anything too quickly. Twinks can damn well pay for anything they want!
The best thing you can do for your newbie friend is advice. Wow is a reasonably complex game with a lot to learn. Genuine newbies are likely overwhelmed by it all.
NEVER give money to lowbies.
Remember, runthrus are a disservice to the newbie and the whole wow community. Stop the rot!