It is amazing how recognizable the Eiffel tower is that eventhough the picture is a bit blur, I bet you immediately know I was in Paris. Even if I say "France", the Eiffel tower would probably be one of the things that come to mind.
For Jakarta, its landmark is Monas, an abbreviation for Monumen Nasional, which means the National Monument.
From the outside, the monument may not be as fantastic as for example the Empire state building in New York. However, Monas is rich in meaning. It was built as a symbol of Indonesia's spirit in fighting for its independence, thus the "eternal" flame on top (coated in gold).
It was also designed with Indonesia's independence date in mind : 17-08-'45. (17 metres is the height of the flame, 8 metres is the height of the room inside the monument which houses the National History Museum, and 45 is the width of the base. See pic, source : http://jakartathecity.blogspot.com/2008/12/monas-icon-of-jakarta.html).
Monas is open to the public. You can go to the top and get a bird's eye view of Jakarta or go to the basement where the National History museum has dioramas depicting Indonesia's history.
When local tourists come to Jakarta, they would most likely put Monas on their list of things to visit. However, for foreigners, maybe less so. I think it is the lack of marketing programme which doesn't make Monas a firm symbol of Jakarta as the Eiffel is for Paris.