So how will you play come release day? Will you play all classes simultaneously, taking turns levelling them to whatever predetermined level you've come up with, or will you play one to the end?
Here's why I ask: When playing the beta, which was limited to level 13, I was finding a ton of useful gear for other classes. On top of that, I was able to easily increase the Blacksmith's level so that by the time I finished getting my second char to level 13, the top items he could make out-levelled me. (The Blacksmith will likely be the source for a good amount of the best gear in the game, especially while levelling.)
So, there are pluses and minuses to playing both ways - and a lot of them depend on your play preference.
If you take turns levelling classes, then here are some pluses and minuses I can think of...
(1) You can use pretty much all the items your other chars find. This would be difficult to do if you try to take each class to 60 in a straight shot since your inventory space is limited enough that you can't store all those tiers of gear for all the classes.
(2) You might be able to get enough gold to consistently get your Blacksmith to out-level you and increase your stash size beyond your need.
(3) Low-level items should quickly get cheaper on the auction houses, so if you are really deficient in a certain item slot, you should be able to get it right away.
(4) Medium and higher level items, particularly mats to level your Blacksmith, should also be cheaper by the time all your chars start to reach high levels.
(5) It could be really redundant playing the same section five times before moving on.
(6) The game becomes really easy for your 3rd, 4th and especially 5th class in your levelling rotation because you don’t need to do much work for the gear. This could be good or bad depending on whether you want the challenge or you just want to get through the levelling process.
(7) It could really undermine the attachment you get from playing one character to level 60 since you are constantly switching between characters, you repeated the same area of the game so many times and you rarely got into any challenging situations with your character along the way thanks to your great gear. You might just end up hating all your characters.
(8) Along the same lines as point 7, it might just be impossible to tear yourself away from your favorite character to play another character you like less or even hate.
(9) And the last one I can think of is that you might miss out on some great "early" deals in the auction houses because gold income at lower levels was too little to afford those items. Then again, you might (luckily) miss out on overpaying for something and use that gold instead for something permanently useful (e.g. levelling artisans and increasing stash size).
Personally, I will create all five classes right away so I have a place to store gear I find for them. Then I will play my favorite character all the way through Normal Difficulty just to get the story and maybe a little into Nightmare to see how it is. Then I will rotate through the other characters in 10-level increments until they finish Normal. In Nightmare and beyond, I will continue to rotate all characters including my favourite until I have maxed out all of them, probably reducing from 10-level increments to 5-level increments in Nightmare and 1-2-level increments in Hell.
The exception might be whatever character I use to play with my friends. I dislike the Witch Doctor – no offense to anyone who likes him; this is just personal preference – so I might use him to play with friends since that will force me to level him.