If you know how to run a business, Diablo 3's market is your friend.  Before we get there, though, lets look at WoW's item progression:  Item Found -> Item Auctioned -> Item Used -> Item Sold/Destroyed/Forgotten about.  

D3, however, goes like this:  Item Found -> Item Auctioned -> Item Used -> Item Auctioned - Item Used -> .... -> Item Sold/Destroyed/Etc.  The main thing to notice is that Diablo 3 has a huge item over-abundance potential, where items are going to depreciate in value as they don't leave the market.  Imagine if TVs were still brand new after ten years of use - eventually even your parents would own a 60" LED-LCD TV.

Now on the currency side.  Gold in Diablo 3 gets progressively easier to find as your equipment improves.  There are gold "sinks" in the game: repairing, auctioning fees, craftsmen fees, etc.  These sinks fight to raise the value of gold, but will not be enough to keep the value of gold from constantly falling.  So now we see the issue:  the value of gold is falling, and the value of items are falling.  The question is, which is falling faster?

The question is, of course, a trick.  The better the item, the greater its usability and the higher its long term value.  Cheap items, on the other hand, will become so numerous as to be unsellable.  But wait!  We have a price floor!  All items can be sold to vendors.  Imagine what would happen if the government said it would print money to buy all screwdrivers manufactured for $4, despite having no need of screwdrivers.  Eventually, every town would have a screwdriver manufacturer. There would be so much money in circulation that it would become increasingly worthless.  Ladies and gentlemen, the market will crash.

The worst decision you can make at the start is spending your gold on craftsmen.  They're expensive, and better stuff is available on the market.   Our saving grace is in the market, however.....is the craftsmen.  They provide a price ceiling.  That's right!  That thing I told you is bad is good.  If really good bows get really expensive, people will enter the market, making really good bows and drive the price down.  It won't effect the really, really, really good unique items, but some of the crafted stuff is really nice.

So I go on, I buy really cheap beginner stuff.  I level up.  I buy price controlled intermediate stuff.  I level up.  I try to buy expert stuff.  Except..I can't.  Why?  The longer you hold onto your awesome item, the more you can get for it, as gold is plummeting.  So why do I want to auction off top level stuff?  So you're left to find your own, or pay out the nose.

So lets recap:  beginner items will get cheaper and cheaper.  Intermediate items have a price ceiling.  Expert items will be ridiculously expensive, especially since every lower level is easy enough to get.  The auction house will make Diablo 3 the easiest game ever to get to a dull grinding drag.

Oh, and you should totally play it.  Its a blast.