well, it
is serious, in that it's the most likely weak point in tor. however, exactly how serious is still an open question.
remember - what the nsa wants is both your location and the site. your location alone only means that you were using the internet (and tor, which itself might be regarded a suspicious). the site alone only means someone was using the site. what the NSA have to do is connect those.
so everything depends on whether the two NSA people can "join up the dots" (graphically, in that diagram, the dark blue dots joining them). because there's nothing "obvious" that tells them that the message with the location is connected to the message with the site.
if they can't make that connection, then they don't know anything useful.
but if they can make that connection, then they know that you (well, someone at your location - an open wifi might give you a little deniability, for example) looked at that site.
the way they might be able to do it is by comparing traffic patterns. if they can show, for example, that every time you send a request, the site receives a request, and if that happens again and again, so reliably that it cannot be chance, then they can make the connection.
so it depends on things like the frequency with which you look at the site, the amount of (tor) traffic when you are around, and the ability of the NSA to assemble and correlate large amounts of data.
[disclaimer: not an expert; i don't know the current state of play on how bad a problem this is; i just know it's a known issue. and if i were going up against the nsa, i wouldn't trust tor alone - i'd use an anonymous, disposable, portable connection device, and keep data to an absolute minimum.]