In rugby there is no blocking. The only time non-ball carriers come into contact with the opposing team is during a scrum or accidental collision (which are quite rare).

Or in Rugby Union, during a ruck or maul.

The main difference in tackling technique between American Football and both codes of Rugby is use of the arms. In Rugby the arms must be used to tackle the opposing player. A shoulder charge, with no arms used, is illegal and may result in expulsion from the game. In a recent New Zealand vs British and Irish Lions game, NZ player Sonny Bill Williams was given a red card and sent from the pitch for this exact offence.

The other main points to note are that the opposing player can only be tackled below shoulder level and, as you've pointed out, in open play only the ball carrying may be tackled (or blocked).

Tackling in American football is almost gone from the game. The object now seems to be to smash into the ball carrier as hard as humanly possible and hope he falls down. You rarely see a receiver or quarterback tackled, you see the defender simply run into him as fast and hard as he can. Watch some film from the 50's and 60's and you will immediately see the difference.

This is because defensive scoring opportunities (turnovers) are so valuable in American football that defenders are pushed to do whatever they can to get the carrier to fumble, and this usually means launching themselves "at the ball" every time.

Tackling the ball itself and not the player.

Or a ruck - clearing out the ruck, it's allowed there. Or a catch and drive in the lineout. But yes, the contact in these situations is more pushing than collisions.

Yes. Far less physical however. You can 'join' a ruck, but not tackle full steam, let alone a shoulder charge or direct hit.

And even the scrums have reduced contact nowadays; arguably this has resulted in them becoming rather difficult to execute well, and they're becoming a bane to the modern game. Tweaking these things does have knock on effects.

I think there's been a positive change since Rugby Union scum engagements went from "Crouch-Touch-Pause-Engage" to "Crouch-Bind-Set". The initial contact is not as violent, and it would seem far fewer scrums are collapsing, especially on the point of engagement.

I don't have stats to back this up, but my general impression over the last couple of seasons is that less scrums require resetting.