Some specifics per continent/country:
The USA is not doing so well. Mexico beat them and currently they are on fourth place. But it is all very premature and everything is still possible, there is only one game played and there are still nine to go. Of course you get three points per game that you won and one point for a draw. The top three of round 5 will automatically qualify and number four will have to go through an intercontinental play-off.
In Europe, the favorites are more or less confirming their status. The surprises are Switzerland (positive) and The Netherlands and Turkey (negative). For Wales and Austria the expectations were also higher. The top two per group after having played three or four games:
A. France, Sweden
B. Switzerland, Portugal
C. Germany, Northern Ireland
D. Ireland, Serbia
E. Poland, Montenegro
F. England, Slovenia
G. Spain, Italy
H. Belgium, Greece
I. Croatia, Ukraine
In South America, it gets very tough. There simply are no real weak teams over there. Brazil is doing very well, but Argentina is dropping the ball somewhat. If they don't improve they will not even make it through the qualifiers since they currently are not even in the top five. (After playing eleven of the eighteen games.)
In Asia, Australia is doing quite well, they are second in their group behind Saudi Arabia.. But Japan is close. In the other group Iran is leading. The top two in every group will qualify, with a round four for the two teams that end in the third position.
There are five groups in Africa. Esp. Tunisia, Ivory Coast and Nigeria are doing very well, they lead their groups. Egypt and Cameroon are also playing great. Morocco is somewhat disappointing, as well as Ghana. I do foresee two African teams getting rather far in Russia...