My main requirements for motherboard are: Intel chipset, discrete, four memory slots, passively cooled and a name brand.
I prefer using Intel chipsets with Intel processors, that way I know they are going to work together with little to no chance of any incompatibility problems. It's something I have stuck to for years which has always worked for me.
Discrete (without a built-in graphics card) is a must as integrated video cards are underpowered and tend to steal processing power and memory that can be better used elsewhere.
While I'm only going to fit two sticks of memory in the motherboard for now, I know from experience that the one thing I almost always do at least once is add more memory at some point. So four memory slots are essential. The motherboard is dual channel architecture, so memory must go in two sticks at a time for the fastest memory performance. In the past I have made the mistake of using only one stick of memory in a dual channel board and it nearly crippled the speed of the computer.
Passive cooling (using no fans - only a heatsink or heatpipe) is a must. Motherboard chipsets are fairly small, so when a fan is used to cool them, the fan must also be small to fit on the heatsink. The problem with small fans is they tend to be noisy, and that noise is typically a high pitched whine which tends to be particularly annoying. They are sometimes temperature controlled, slowing down when the machine is idle and speeding up as the temperature starts rising - and a change in a fan's speed will usually be more noticable and annoyng than a constant, but higher noise level.
The name brand thing is for a very good reason: You know what you are getting and the support is usually good and continues for years. I have an eight year old Asus board in one machine and they were still updating the bios a year ago. Also, when the inevitable happens and XP needs reinstalling, it's easy to go to the manufacturer's site and download all the drivers needed. I've found that with a cheaper, no-name board it can be impossible to find the drivers or updates (even if you find out who the manufacturer is).
So I started looking at the following brands: Asus, Intel, Gigabyte and MSI.
Intel are about as stable as you get: good solid boards that last for years and are not prone to any kind of stability problems. My last three builds have been Intel. However, they tend to be conservative boards, running a little slower and they don't allow overclocking (the last not an issue for me).
Gigabyte and MSI both offer boards with good features for the money and are also fast and reliable.
Asus tend to make the fastest boards which are very long-lasting stable and overclockable. The one I'm looking at is the ASUS P5K, which has a heatpipe on the Northbridge chipset, with the radiator positioned right where the rear case fan will be. So the chipset should be kept nice and cool, even though there is no processor fan.