They really are pretty much the same when used as prepositions. Like you say, that's when a noun or noun phrase follows them. (The -ing form of the verb in this case acts like a noun).
I like all vegetables except carrots.
I like all vegetables except for carrots.
I like all sports except skiing.
When you use 'except' as a conjunction, you can't replace it with 'except for'.
All the vases look the same, except (that) one of them is cracked. (Here you CANNOT use 'except for').