cos DOTA is all about team playing, when something goes extremely bad, people would start blaming it on the teammates, and noobs will always get picked on first.. most frequent complaints:-

1. stun too early; stun too late
2. ulti wrong
3. no backup; always farm nc
4. feeder

usually blames are on heroes that requires a certain degree of skill to use like SF, riki, techies, sand king etc.