agnok
Could you give us a few pointers on what convinced him you were a good fit... Would help us third rounders..
Well in my case it had to do with my interview. I think I was just very enthusiastic, spoke pretty clearly, and was able to explain complex technical things in a non-technical and entertaining way. You have to remember - at Harvard, you have to be able to take a position in front of a class of 90 people, speak your mind clearly, and back up your position confidently. Additionally, you will be an expert in one or more fields, but many of your cohort won't be. So you have to be able to communicate your points in a manner that everyone will understand.
This same concept applies to your application essays as well. So if you're technical, or have strong IB experience, or something else in depth - you must be able to communicate it well in general terms.
Hope that helps.