AA guns tend to reduce the value of "strategic" bombing. Hence you would do this more (and spend more researching heavy bombers) than in other versions of the game.
There are many players that believe that heavy bombersheavy bombers unbalance the game, and one should not be allowed to research them. If they are disallowed, AA guns would be less necessary, and can be eliminated from the game to simplify it.
The construction of factories is most important for Japan and Britain, who start with only one (everyone else starts with two). Hence, it makes these two countries harder to play.
There are some problems with the Factory RuleFactory Rule. The first is that one costs a "flat" 15 IPCs to build. But that doesn't reflect the fact that factories are easier to build in some places than others (e.g. in "real life," it's almost impossible to build factory in Sinkiang). Second, most countries would not "export" aerospace technology, meaning that realistically, it would be possible only to produce infantry, but not airplanes in a foreign factory.