So in a way it's harder to maintain a high position than to gain one. If you are the top team you won't get as much points for beating the nr 20 team on the list as the team ranked 10 would get.
This is wrong. You get the same number of points for beating a certain opponent (say a nr 20 team) no matter whether you are ranked 1 or 200. You do get more points for beating a team ranked 20 than a team ranked 100 though.
The main issue with the current system is that your points for each year are the average points per match (5 match minimum). This is an issue because winning 20 out of 20 matches in a year is effectively the same as winning 5 out of 5. Spain's recent long run doesn't really help too much because it's only 18 months of results, which get's averaged out. Rankings are based over a 4 year period, so being decent for 30 months and amazing for 18 months isn't as good as being very good for 4 years.
The strong nations in the weak confederations tend to be overranked because they play a lot of games against weaker nations (which they win), and very few against poor nations (which they lose/draw) - but because there are so many more of the former, the average is weighted towards the games against the poor nations. Of course, because they are overranked, in future this means that teams get even more points for beating when these teams play each other, because they are overranked. USA and Mexico seem to benefit like this - both are about equal strength and beat eachother a lot, for which they get a load of points - which helps maintain/improve such a ranking.
The current system also awards more points for World Cup finals matches (worth quadruple points - it's like a game show, isn't it?). Because weaker nations are usually given places in the world cup that they don't deserve (in the sense that they aren't near the top 32 nations in the world), they get bonus points that they don't deserve either.
Essentially, dumb system for ranking teams. One might be tempted to say that's the point though - top nations don't really care (they just want to win tropheys) but smaller nations have something to play for. If you boost the ranking of (say) the USA, Australia and Japan, you help football develop in those areas, so it's in FIFA's interest to come up with a "bad" system.