Well, after having gone though different strands of the video lecture of Dr Zakir Naik and an article written by Ibrahim B. Syed (the president of IRF and Professor at the University of Louisville, Kentucky), it should be authenticated to say that there are six criteria for observing Hijab as already mentioned by a fellow member in a post above. Though, covering of face and hands is not proved through any Hadith or a Quranic verse, etc. All the Hadiths (given by some so-called semi-skilled Mullahs) proving that the covering face and hands of a woman is also an obligation, are mandatory, self-fabricated and unauthentic. It is therefore a cultural option and not the commandment of Quran to cover the face and the hands. When the lady companions of our Holy Prophet (PBUH) would expose their face to others, then what more is left to say to prove the same.
Given below is a video lecture by Dr Zakir Naik to throw light on the topic under debate.
HIJAB DR ZAKIR NAIK - YouTube

The following article is richly informative on the topic of Hijab.
Women in Islam: Hijab

As far as the question whether it is fitting for a women of the modern world to observe Hijab is concerned, in my opinion it is just a diabolical nonsense to say that women cannot move in sync with this fast-paced modern world by observing Hijab. Hijab can never hamper it in anyway.

The following link is interesting as well as helpful for the same as it carries a moral lesson for both male and female regarding Hijab.
Haya Aur Parda - A Message For Muslim Men And Women.. - YouTube

Regards

__________________
Real richness is that you are so expensive that no one can buy your character.