Magic find is one of the best stats to have if you are looking to make money with the real money auction house, or just to gear up your main character. Blizzard has promised that Inferno mode (Diablo 3 endgame content) will be very difficult. This means that it will be difficult to stack magic find as opposed to damage and health stats. Blizzard has also recently changed the way magic find works with party members. The total magic find of all characters in a party will average out for each player. This prevents one player from having to stack magic find gear and share all the loot with other members of their party. 

In my opinion, the Witch Doctor is going to be the best class to do magic find runs with solo. The Witch Doctor is a range class  with plenty of crowd control abilities that will slow enemies, giving them the ability to "kite" some of the harder mobs and bosses. Range classes will have a much easier time killing harder mobs and bosses solo. Witch Doctors also have pets that will help tank mobs and bosses so that they have time to deal plenty of damage without getting hit. Each class in Diablo 3 has their own passive and active skills, as well as runes that will modify each of the active abilities. You can use the official skill calculator on the Diablo 3 official website to practice building your characters. Since Diablo 3 is still in beta, some of the abilities for each class might change before the game is released.

Magic find runs will be very common in Diablo 3, especially with the release of the real money auction house. Remember that grouping up with other players will make the enemies more difficult but you will also gain buffs and have access to much more crowd control if you mix and match different classes. Be sure to group up with other players with at least some magic find when doing MF runs so that the average is still enough to get you some loot. Hopefully all the classes in Diablo 3 will be somewhat balanced but as of right now, the Witch Doctor seems to be the best class for magic find runs. This information is based off of the current skill calculators and beta gameplay. Check out the official Diablo 3 skill calculator and see which class you think will be best for magic find runs once the game is finally released.