Oooooooh. Good point. that line always sticks out so I'm gonna do some thinky thoughts. Loki, as a villain needs good lines or at least catchy, memorable ones. He had lots in Avengers, so I suppose one of the reasons for that line is - you need to give those to that kind of villain. I think. I'm not a movie expert. I'm just going by things that I appreciate in movies.

But if we consider this as a part of narration - and many details in Avengers were subtle, precise and very deliberate - Loki is god of lies. A god of lies, in order to lie well, needs to read people. He is few thousand years old and has tons of experience on judging people quickly, he sees how Clint reacts and does his math - that guy is reliable, loyal, efficient, and most certainly deadly. It's also possible that Loki, thanks to Tesseract power simply sensed some things. "You have heart" proves to be true later - I mean, Clint wakes up after all that mess, picks himself and his bow up and goes into fight. He wouldn't be able to do that if he didn't have lots of heart. :D