I believe there are three types impressive people:
1. Impressive Right Away
They are the most obvious. They try very hard, and although some do succeed to impress you, it's not long lasting.
2. Not at All Impressive
No matter what they do, there is no way that you click.
3. Impressive Over Time
This can at first be confused with the second group. However, this is the most genuine, and get only better over time.