What qualities does Montag have that makes him receptive to Clarisse's influence in Fahrenheit 451?
From the way Montag responds to Clarisse McClellan it is clear that what makes him susceptible above all to her thoughts and ideas is his kindness and his protective nature. When he sees Clarisse, what stands out to him most of all is her sense of fragility, and it is this that rouses his protective instincts. There is also something strange and different about Clarisse that clearly makes Montag intrigued about her and what she has to say. Note the following quote that comes from their first meeting together:
Her face, truned to him now, was fragile milk crystal with a soft and constant light in it. It was not the hysterical light of electricity but--what? But the strangely comfortable and rare and gently flattering light of the candle.
Note the emphasis on Clarisse as being "fragile milk crystal." In particular the description of her face as possessing its own light that is compared to the soft light of candles is something else that draws Montag to Clarisse and clearly makes him think about what she has to say. The qualities that Montag possesses therefore are based around his protectiveness towards Clarisse and also his attraction to the strangeness of Clarisse.