real life physics + Avatar = complete confusion Back to the current topic: Flying with firebending is unusual. Everyone expects airbenders to fly, but firebenders?! Azula's skill is something that will catch you off guard.
Also, metalbending isn't as easy as it seems. Metal is purified earth, so it takes the very best earthbenders to sense the "earhly" elements in the metal. People have probably thought about it, but were not skilled enough to do it. BTW, Toph had not been a master of earthbending for 12 years. She's twelve years old now (-_-). Plus, she never really needed metalbending until the end of season two.
Why do you think all waterbenders should know that you can bend plants? At the North and South poles, there isn't much of a need for plants to bend since you would be surrounded by water and ice. Most waterbenders probably never needed the skill, so it was never thought about. Notice the waterbenders who did not live at the poles knew about plant bending. They needed to know the about plant bending in case there isn't a large supply of water around (unlike the poles where there is enough water always). And bloodbending was simply not thought about. Did you think about bloodbending before it was mentioned? It took a bit of thought to realize that you can bend humans because it isn't ordinary to think about waterbending in that way.
The air nomads were ambushed. The fire nation probably had thousands of soldiers. Plus, there were mostly kids at the air temple (as far as we know). The air nomads weren't going to win a fight under those circumstances. Plain and simple. And did you notice that there were a lot of fire nation skeletons and helmets lying around at the Southern Air Temple? You can infer that the airbenders did indeed defend themselves.
It wasn't that hard for fire nation soldiers to get to the air temples once they found out where they were. The air temples were on mountains. You can climb mountains if you have the right equipment. Problem solved.
