Well you read the laws of the game and it states and as El Grap says, the key words seem to be careless, reckless or excessive force and the other key phrases seem to be acted with complete disregard for the safety of opponent i.e. yellow and is in danger of injuring the opponent i.e. red.
When you describe a slide tackle, you can make a slide tackle in a perfectly fine manner and execute it with skill but I have seen a lot of slide tackles where the studs are showing, despite getting only ball, be judged as a foul. I have seen more than a few 'normal slide tackles' been whistled up.
Hence my question. You kinda need to see it before you can judge it.