He made conservation accessible to everyone, and made people here (in Australia) think about the environment. Until the mid-60s we only had The Bush. Then Vince Serventy, Rolf Harris, and The Leyland Brothers started making documentaries and suddenly we had Nature. Through the 70s and 80s we got an Environment, but the average bloke saw environmentalists as either scientists in lab coats or fluffy tree huggers.
Then Steve Irwin turned up. Everyone saw a bloke like us in khaki talking about conservation and saving the environment. He made people care about the animals, even the ugly ones. He saved patches of bush, even the useless stuff like rainforests and swamps. He got everyone thinking about the world we should be caring for.
I'm going to miss him.
Douglas Adams wrote in "Last Chance To See" that losing a species makes the world a poorer place. I think that would apply to Steve Irwin.