To understand the motivations to purchase bitcoins, I would simply take a look at where people spend theit bitcoins. Which bitcoin-only businesses are currently booming?
1. The gambling industry
2. The drug marketplaces
This probably changes in the future when bitcoins start replacing Paypal. However, currently those are the two industries driving bitcoin adoption.
Pretty good take on things. A lot of the gambling, however, comes from bored speculators IMO. I don't know of many people buying Bitcoins to play Satoshi dice, but plenty of hoarders will play since there is nothing better to do. This isn't to say there isn't some gambling going on where people are acquiring to gamble.
You'll notice the areas where Bitcoin thrives is where it can vastly outperform the other options. Things like online retail just aren't that useful compared to the options. A lot of people complain about the credit card fees that exist, but that gives you a lot of protection as a consumer and the cost of acquiring Bitcoins quickly makes it not useful for the non-Bitcoin hoarder.
For Bitcoin to succeed, it will first be in places where it vastly is superior. Drugs and gambling it currently is vastly superior. I'm surprised we haven't seen more on the porn side as well. Once you have more widespread usage in these areas, expect the cost of acquiring Bitcoins to drop, thus making general economic activity more common. It's a bit of a chicken and egg problem - you don't really use Bitcoin as a currency if there is nowhere to spend it, and you don't accept it because not many people are spending it with you. If it starts becoming more widespread (say 10% of people use it for something, even if its just some niche use), then they will be much more willing to accept it. Then small edges become more useful (sending money with Bitcoin to a friend would be easier than PayPal, etc..., but not if your friend has to go through an exchange to spend it or find some small site to spend it).
Find areas where people cannot buy things with traditional means (gambling, drugs of course, but what else?), and that's where you'll see it take off.