I have to agree with Sean's two points made above. I think the wealth of knowledge made available through websites like this, YouTube, and the blogs of throwers has just been a great tool for throwers/coaches of all divisions and all levels.
Related directly to D3, I think we've seen the competitive level in many conferences rise over the past couple of years because of small pools of talented throwers competing against each other each week. I know from experience, having an in-conference competitor like Sean, and his teammate Judd, produced a much more competitive environment. To be one of the best in the conference, you had to post marks that were competitive at the NCAA level as well. This has been the case for the WIAC for many years now, and I think we're beginning to see the same effect on many conferences throughout the country as well.
Another factor might be the success of post collegiate D3 athletes. Seeing former All-Americans and National Champions compete with the country's, and sometimes world's best, has given quite a few athletes hope to compete at that level when their collegiate career is up.