For those complaining that Robinson was just going in for a hit, that Maclin ducked into the hit, etc, etc. take a look at this video and see how you properly tackle someone:
http://www.youtube.com/watch?v=yhpaMxvvAwg
Now, this happened before "defenseless player" penalties were enforced, but you can see this could fall under that category. There's even some helmet contact, but I don't think this is deserving of a penalty (unlike Robinson's). Here are just some differences that I see:
1. Willis is using his arms to wrap up. His goal is clearly to TAKCLE Bush, not knock him out. Robinson's hit, there's no arm motion whatsoever. He just lines up Maclin, and throws his upper body (shoulder, head, whatever) into Maclin's head/upper chest.
2. Robinson is still standing after the hit. When he's leading with his shoulder/head and still standing, it's impossible to hit Maclin anywhere lower than his shoulder/neck area. P-Will aims for Bush's center of gravity, and this of course means he has to aim much lower, which means he's diving.
3. Bush lowers his head much more than Maclin does, and there's definitely helmet-facemask contact, but it's just grazing. This is what happens when you aim for the lower body.
4. Related to #3, you can see as the top of P-Will's helmet grazes Bush's facemask, P-Will is actually raising his head up so that he's looking at Bush. This means he's not using his helmet as a weapon, he's looking to see where Bush is to make sure he has a sure tackle. As they go to the ground, P-Will is staring at Bush's face. Completely different from Robinson, who's not looking at Maclin at all, just looking down and hoping he delivers a big hit.
5. Finally, and maybe most importantly, P-Will is fired up for making a great play and both players get up uninjured and ready to play more football.