Portability - Look for a stand that's lightweight and easy to carry around, otherwise you'll likely use it once and consider it far too much hassle in the future.
Adjustable height - There's no point swapping one uncomfortable set-up for another. Make sure the stand has adjustable height settings to provide comfort for your posture, no matter the position you like to work in.
Compatibility - Check that the stand is compatible with your laptop's size and weight. Some models also offer adaptable elements that can work for iPads, too.
Stability - This one's especially important if you're accident prone or work near those who are. Look for a stand that has a stable base to prevent wobbling and potential accidents.