Whoo boy, do cases matter! Once upon a time I thought, eh, a case is a case... get the cheapest.
... and then I started building computers.
Aluminum cases are FANTASTIC if you are going to be hauling the thing around to any degree at all. I am MUCH more willing to tear into an aluminum case than a steel case if there is a problem. I hate steel.
Some cases open up like a well juiced nymphomaniac, others are harder to pry open than Jimmy Hoffa's grave. The problem here is that you have to work with the damn things to get an idea which is which. Dell's, for example, are very easy to open up
You need to be sensitive to the case's form factor (the size of the motherboard that it will accept). Some cases are flexible and will let you slip in most anything, others only accept one size... usually small.
There are cases that work especially hard at keeping cool, cases that incorporate filters (if you work in notably dusty environment, a good thing to have), cases that are HUGE, cases that are tiny, cases that can be rackmounted, cases that can't.
Another place where I've been bit is in how many internal drives (i.e., HDD) you can mount. I've seen cases where I can slip in six, and cases that only hold one... which was a major pain-in-the-ass and required the case to be tossed when expansion was needed.
A lot of people get hot over power supplies, but I have used, mostly, plain jane whatever came with the box and very rarely had a problem. Power supplies do fail, but I have had worst luck with keyboards. Do NOT get one that is too small (i.e., >300) as you are just begging for trouble. The more peripherals you plan to cram into it (oh, yeah man! I've got six HDD, four DVD, and fifteen PCI cards running....) the more juice you are going to need. Generally speaking, unless you plan on running a server anything between 350 and 400 watt is not likely to disappoint.
Note that some power supplies specialize in being quiet, and you will pay for that.
Luck.