How far ahead can I skip in multiplayer games, and are there any disadvantages to doing so?

As usual, some of my friends are racing ahead through the story while I'm stuck at work earning a living. Now, I'm aware that you can't join somebody who is playing a later act, but what about joining somebody at a later point in the same act?

In Diablo II, you could skip straight from the beginning of an act to the last boss if there was somebody else in the game to portal to. Has this changed in Diablo 3, and if so, what are the new rules/mechanics involved?

Solution 1:

Once you have unlocked a difficulty level, you can play any act/quest on that difficulty level if you join a friend's party. (i.e. With a brand new character, you can join someone on Normal Act IV with no problem, but you can't join someone playing playing Inferno act I).

As for mechanics, if you haven't unlocked an artisan (such as the blacksmith), joining a quest after that artisan is unlocked will instantly unlock them for you.

The problem you run into with jumping straight to later quests is the exp penalty when playing with characters who have a major difference in level. You are expected to be playing with other characters within 5 levels of your own, and if you aren't there are experience penalties for the lower level character if the characters are more than 5 levels apart.

Solution 2:

You can skip ahead to later Acts. I was partway through Act II, and my wife who had just killed the skeleton king joined my game. She was shown the Act I final cinematic, the Act II intro cinematic, and then entered my game on the same quest as I was. She hadn't completed Act I on any character before.

This leads me to believe that you can join a friend at any point in the story, regardless of what you have accomplished prior. This may be restricted by level and/or difficulty (will edit if I learn the specifics).