RTR.
When manufactures switched over it really changed the market. It means a teen can save up a relatively small amount of cash, buy a Stampede and run it the same day. It also saw a general increase in the availability of parts as more model shops took the risk of stocking high value RC cars because they could just sell a complete box set up without having to have the knowledge of every single product. RTR has caused a massive shift in the RC landscape and I can't see it ever going back.
Fibre Reinforced Plastics
Pretty much every RC now uses them, there was a time where they were reserved for the A-Arms of racers who didn't want to use expensive graphite arms. The drop in cost of these plastics as well as developments in the area mean we are seeing stronger and stronger cars. I mean we've all seen cars going 15+ feet in the air and landing perfectly fine, for their scale that is incredible. That wouldn't be possible without the materials and manufacturing developments that are now common place. My old AE buggy used to eat arms with a bad turn, a flip could often mean a broken shock tower, now I don't even flinch. The Internet
Forums like MSUK create a community that drive the hobby forward. We can search the web for answers to our RC questions, places to meet and local groups. I stumbled across RC by accident as a child with my dad when a village hall was double booked on the day of my birthday for my party with 5 or 6 guys carpet racing. The exposure the internet allows, the driving down of prices and the general access it gives the consumer drives the hobby. We all buy from Modelsport, Hobby King, AsiaTees, Dollar Hobbyz to name a few....In fact the last RC I bought came from Switzerland. It allows RC brands to works faster and more efficiently. I know it's not specially just an RC thing but it really can't be overlooked. It is where our main community lives. I mostly drive on my own, but I am never alone as I know there is a community out there that is really awesome. A hobby is nothing without the people.