Also so many of the calls that get people upset are judgment calls. I can't believe people think replay would be clear and non-controversial on fouls and PK calls. There are no hard, clear rules that a replay ref could seize upon. You'd just be substituting the judgment of someone in a booth for someone on the field. The only foul calls that would be subject to clear review would be those based on a total dive, where the "fouled" player goes to ground without being touched at all. If the player was touched, but arguably not enough for a foul, it's all judgment. If getting the ball were a defense to fouling someone you could review that, but it isn't (except sometimes it is clearly a factor based on, yes, judgment. The difference between foul, yellow and red is pure judgment. This is exactly why holding and pass interference calls are not subject to review in football. There is contact, but was it enough?
Goal line technology works great. It's a shame it is so expensive. The ball is either past the line or not. We could put cameras on the end line and use replay for disputed goals but there really aren't that many. Offsides is theoretically conducive to replay, because the player is either beyond the next to last defender or not. But the necessary system of cameras to allow review of offsides calls would be ridiculously huge. So what are we going to review here?