CSS compatibility and support for Browser.
If you are working as a Web Developer you must know about browsers. If you love to work with your favorite browser and you focus only a single browser for work then it can create hell lot of trouble. If you love to work on firefox but your client uses IE then what???
You would create a bad impression and not only that again your project goes for corrections.
Same way if you are thinking to shift to css3 first take a look at the browser support then you can have a go. I found that my 40% clients are still using IE6(no idea why??) so it will take a long time for me to start working on css3.
Lots of properties are still not supported by IE.
10 Useful CSS Properties Not Supported By Internet Explorer
According to statistic still more then 10% of people uses IE6 and latest browsers are not properly ready for css3, So before starting CSS3 take a look on browser support for css3.
CSS 3 selectors Specifications Chart from quirksmode.org