One of the useful features in Yahoo and other messengers is that the ability to set your status as invisible. This feature is necessary when you want work with out any disturbances and also see who is online. Google Talk doesn’t have this feature, but luckily Gmail chat got this feature now.
This feature is a much needed one for Gtalk, Google is very slow in adding features to Gtalk so we may not expect this in the near future. Via Google Blogoscoped