I've been running meet-ups and speaking at and attending conferences of all shapes and sizes all over the world for years and I assure you, unless you're exceptionally talented and experienced at public speaking (which you may be), then a lack of preparation is painfully obvious to the audience.
I completely agree that building a bond with the audience is the key difference between a good talk and a great talk but to anyone relatively new to public speaking, I promise you that there's no such thing as too much preparation.