They took a great classic gametype that has been in MM since Halo 3, but was ultimately played first via honor rules in Halo 2, and dumbed it down. They made it too hard for the humans. The Flood lunge in combination with the thruster pack is absolutely insane. The flood not being a single headshot kill is also another huge problem.
Overall I was very disappointed with the way that 343i handled Infection in Halo 4. It had so much potential, but in 343i's hands it was a colossal flop.