Where is the airspace not covered by primary Radar?
There are a few spots not covered by primary radar. Here are a few I can think off:

Afghanistan, not really what you'd expect from flying between Europe and South East Asia. I was told though by the pilots that the Americans were keeping an eye on us. I'd imagine there are a few holes in the CIS countries as well.

Multiple areas in Africa, but this is improving I believe. There's a post on PPrune. Apparently known as the 'dark continent' for spotty coverage and consistency. South Africa has good coverage, and I'd guess the same for much of northern Africa as well.

A few remote areas in South America, then again, improving.

Oceans are not covered.


Determining factors are population density, aircraft density and economic development.

As more you Malaysia map, if there's a radar at Kota Bharu [KBR] airport, here's the map assuming a range of 200 Nm for the radar. It should in theory be well within reach.

(source: gcmap.com)
And an extended version:
SGN Ho Chi Minh City (Saigon) [Tan Son Nhat Intl], VN
KUA Kuantan [Sultan Abmad Shah Airport (Pedang Geroda Airport)], Pahang, MY
KUL Kuala Lumpur (Sepang) [Intl], Selangor, MY
KBR     Kota Bharu [Sultan Ismail Petra], Kelantan, MY

(source: gcmap.com)
Just for the interested, here's a map showing Japanese military radar, just to get an idea of coverage in a country full of people and planes.

Maps from http://www.gcmap.com/