It doesn't give me that impression. It gives the impression that you have been thanked just for reading the post, and most likely get more thanks after trying to answer it.
BTW, the time to say "thanks" is afterwards, not beforehand which gives the impression that you take other people's voluntary unpaid assistance for granted. Or as British politician Neil Kinnock put it, "Don't belch before you have had the meal." Prefer to use "please" beforehand and if you find a response helpful then you can use the "Thank User For This Post" button.
That is really simple. I've always seen:
var x = document.getElementById("mylist").value;
and have used it out of habit.
var x = document.getElementById("mylist").options[document.getElementById("mylist").options.selectedIndex].value;