Definiteness marking is certainly an area where speakers of languages that don't do that get confused. But I'm not comfortable saying that there are a ton of places in English where you can choose to say "the" or not. In most cases, there's only one correct choice.
Going back over the nouns in my previous paragraph: (marking area[0] speakers[1] languages[2] I ton[0] places English you cases choice[1] [there])
[0] rival article already marked
[1] "the" permissible; presence and absence are both fine
[2] "the" possible; presence may raise eyebrows
The problem with o- is that both men and women use it, but women use it slightly more, and getting that right is hard for a non-native.
The plurals is other dead giveaway. Apparently articles and plurals are very difficult in English, but to me--a native English speaker--they are the most natural thing in the world.
Of course I'm complete shit at the Latin-based languages because I can never get the masculine/feminine words right.