DO NOT GET A GE or HOTPOINT MICROWAVE!!!!
There, that's my advice! If you do, it will break on you. :stickbeat
Over the range microwaves in stainless steel are very popular as are black finishes. If an over the range microwave won't work for you, then you really need to consider how much space you want to lose and what you will be cooking in it. If you only use it to heat water for coffee and cook popcorn, you can get a smaller microwave. If you are like me and like to nuke three bowls of Top Ramen noodles for your kids, then you will want to go with something larger.
I am a fan of whirlpool appliances, but that's just me. Of course wattage is important, but I wouldn't waste my time worrying about a certain model being powerful enough. Look for something you like first and check wattage last, most microwaves are powerful enough to cook anything quickly anymore.