Hmm, all kinds of things: excitement, suspicion, lol...
First of all, I don't take people seriously (especially if they are joking or drunk or 'totally inappropriate for me' or I don't know them that well/they don't know me that well...) There may be shock and 'WTF?'/'OMG!' factor too lol...
If I do take them seriously, I later start wondering, would we be compatible at all (and usually convince myself out of it all, lol!!) I look for signs that we would or would not be compatible, from behavior to opinions to wishes and situation in life etc.
It depends if I'm attracted to them or not and how attractive they are (not just physically, also psychologically lol - someone can be hot-looking and a jerk or chauvinist lol) Some opinions and/or behaviors can be definitely unattractive...
I'm sometimes surprised what sort of things people appreciate me for too lol... (eg for being different from their exes and not blowing up in certain situations etc lol)
And sometimes I've been surprised people maintained interest even when I did blow up at them, hm. People are weird, sometimes
So, there's hope for everyone??