A key position is a country that (may) have a big strategic and/or tactical advantage.
Beside the five capitols and WUS the following areas can be considered as key positions:
Eeu is very important for the axis. If the allies can hold Eeu permanently the fall of Germany is most likely only a question of time. Under specific circumstances Germany can be forced to transfer Eeu temporarily into an exchange country but the axis should calculate thoroughly if they are able to recapture and hold it or if they are able to cause high damage by strafing Eeu. However using Eeu as an exchange country carries a high risk.
Len is very important for the Allies in order to transport replenishments from UK and USA to Europe. If the Axis gain control over Len ? and more important ? can hold it, then it will be very difficult for UK and US to support Russia. If possible, Len should be under Allies? control from the beginning.
Nov is for both sides an outmost advantageous position, if fortified sufficiently. The Allies can keep Japan on distance for a long time by playing cleverly (maybe as long as it takes to win the game!). Furthermore you can avoid that Eve and Kaz become exchange countries and instead use Yak and Sin by what additional income for Rus and US can be collected. But on the other hand, if Japan is able to hold Nov then Japan will have a high income and Moscow is threaded directly. The Allies should fortify this land with additional units if Japan has many bomber units available, because Allies could loose a complete front at the time Japan develops Heavy Bombers.
It is very important for Germany that it controls WEU and it should be only made to an exchange country if recapturing can be accomplished without great losses. In that situation you should prove if the German troops would eventually get into a dead-zone.
Very beneficially for the Axis to have. The major part of Africa can be controlled from that point. Utilizing the transporter support line, Japan can build up enormous pressure if planned accurately. The Japanese fighters have a very strong defensive position in EGY since from there all important European countries are in range. If a British attack takes place in a European country the fighters can support the defense in order to prevent that US or Russia can take control over the country. Furthermore the Japanese air threatens allies? ships in AZO as long as WEU is controlled by Germany. At the time EGY cannot be hold anymore all Axis? units should be withdrawn from Africa the turn before US and UK could attack. It may happen that with this move the pressure on Europe is maximized in a way that the Allies? units in Africa are suddenly useless.
Advantageous for both parties. The allies are able to proceed from that position to Africa and southern Asia in order to gain an IPC advantage. On contrast Japan can send units to Africa every turn without having transporters. Furthermore Japanese fighters stationed in PER have a similar optimum position as in EGY. In fact they are not able to attack AZO but in return NOV, YAK, EVE and SIN are in range. However precaution has to be taken if allied tanks are present in MOS or LEN. If KAZ or CAU is cleared by US or Russia, Russian or Britain tanks plus air force may destroy the Japanese air fleet.
A good place for an axis? industrial complex. Tanks placed in IND can be in Africa quickly. However the industrial complex should be defended adequately, if allied tanks are present in NOV, KAZ or CAU. Especially if US liberates SIN or PER then Russian tanks would be able to attack IND. If IND is taken by Russia UK would be able to produce new units instantly and if the defense in IND would be supported by additional tanks and fighters then Japan would be in a very disadvantageous situation.
Useful for the Axis if controlled permanently. With YAK under control direct pressure is on NOV and most likely EVE and maybe NOV can be made to an exchange country.
Very useful for the Axis. Similar situation as with YAK, but controlling SIN gives more room for expansion. Africa is closer and in addition by controlling SIN the access for the Allies to numerous countries is blocked.
Disadvantageous is that supply takes a little bid longer to SIN as compared to YAK.