The advantage is the modularity.Hervé said:Actually D20 is exactly that: one special rule for every situation, every ability, feat and so on
Only if you have any given ability or feat, you must pay attention to the rules expansion thereof.
This makes a very comfortable learning curve.
While for some special cases the rules really are "clunky", in 99% of all cases it's all smooth.
In very specialised cases it can be interesting to reach a consensus about how the rules are to be interpreted.Too many rules lead to absurd situations and this very thread is quite a good example about it.
But usually this only deepens the understanding of the rules.
Personally I consider D20 to be an elegant system.
D20 is not my ideal of a RPG system.There are many systems that allow to handle such various situations with far less maths and using a common method for task resolution.
There are a lot of things I don't like about it, but it can be fun to play.
I don't want to start a religious war, but which systems do you mean?
The systems that get mentioned here from time to time don't seem fitting.