Originally Posted by Calvin
I am not convinced that Ng5 is bad. Take a look for yourself. What does black play here?
Correct play for Black is to respond to 4. Ng5 with 4. ...d5. Best play is 5. exd5 Na5, forcing the bishop off of the a2-g8 diagonal. White can either retreat immediately with Be2, or give check on b5, which only delays matters after 6. ...c6 7. dxc6 bxc6 8. Be2. Black should then proceed to kick out White's knight with h6.
After this position, White has a pawn and no weaknesses. Black has an out-of-place knight on a5, but has many open lines and files to use.