If you play in windows, you'll be able to have non-accelerated mouse movement (can also be achieved in Os X using USB Overdrive, although you cannot have very high sensitivity and no acceleration in Os X unless you have a very high resolution mouse, 2000dpi+), you'll be able to play in windowed mode and to play with LAN latency on ICCUP if you use 3rd party applications. Also, Starcraft will take up most of your CPU even when idle, while the windows version can limit how much CPU SC will use.
If you're only playing casually and don't mind about mouse movement and don't play on ICCUP, there's not really any disadvantages using Os X.
As for playing on a laptop in general, the fn key will really screw you up in terms of unit grouping, though if you always play with a laptop keyboard, you'll get used to it eventually and it won't limit you much.