It is hard, perhaps, but a good attack for the NSA would be to run many of the exit nodes.
The intelligence gathered this way would be very valuable, as the traffic on the TOR network is has a much higher intelligence value. This is because it is used by those trying to hide something, something which the NSA may like to know.
If I wanted to support the NSA, I'm sure I'd do volunteer work for those organizations -- and if I ran an intelligence agency, I'm sure I'd recruit assets off university campuses across the globe. Just saying.
The intelligence gathered this way would be very valuable, as the traffic on the TOR network is has a much higher intelligence value. This is because it is used by those trying to hide something, something which the NSA may like to know.