When I play against someone in person, I love glancing up at my opponent and seeing his or her reaction to my moves and the flow of the game. An interesting moment in the Alphago games, actually, was when Lee Sedol glanced up at Aja Huang as to see the expression of Alphago. Of course it was nonexistent but it was habit to him.
I don't get the opportunity to play people in person too often so I don't have that kind of experience just yet but there is also a similar feeling playing online. Sometimes humans make mistakes, overlook certain things making it your job to capitalize. The exhillaration that comes with realizing you can win the game because you saw something your opponent didn't is indeed fascinating.
Playing against a computer has a completely different spirit, the way I see it. Part of that I think has to do with the sheer way that they play. There is an opening book that has been programmed. A joseki book, perhaps. After that, it's brute force for the most part and in the case of Alphago, a neural network, to evaluate the position and make decisions. Emotions and situation don't effect the way a computer plays. Computers don't necessarily take risks like a human would.
There is a sort of disconnect between the players. Go is a game of two players (or two sides in the case of Rengo). Playing against a computer removes that aspect of Go. Now one is not playing against someone, the opponent is now something.
What turns me off the most, however, is the lack of creativity. Alphago did play some very innovative moves. However, at any level lower than that, there is not much creativity that I've seen. Stronger players obviously will have a better sense of this than I do but that's what I believe is the case at this point.
While playing against computers is beneficial at the start, I feel it takes a bit away from the essence and beauty of Go; the spirit and thought; the environment and mindset.
Let me know what you guys think :) I'm always open to more thoughts and ideas! Thanks for reading!