Time to payoff
Immediate, short, and long term payoffs
Some items are in such demand that they can be farmed very quickly and sold quickly, if priced correctly. This is a very fast way to make money, though not necessarily the best as there is almost always intense AH competition among suppliers for quickly/easily farmed items.
Other items may take longer to sell.
And yet some items may take ages to farm, but fetch a handsome price. Examples would be anything with a very low drop rate and a ready market. Pets are a good example. There are many which have a drop rate of 1 in 1000 or worse – and yet there are buyers out there for these. They will usually fetch a very good price. There are some risks with the long harvest strategy, though, among them that a) for whatever reason, others enter the market and pursue the same strategy with the same item (e.g., let’s say you got greedy and listed a dragonling for 8,000g; this can be a cue to others that there are people willing to pay crazy prices, and they might then go farm the same item and undercut you by a lot, thus making their “cheap” price relatively attractive), or even b) that Blizz might increase the drop rate (thus making it easier for others to get the same item and increasing supply in the AH).