/snip
#2
DM: Okay, after leaving you trapped beneath the earth by the landslide, your archnemesis has escaped. You are eventually going to run out of air, and if you don't stop him, he will have the McGuffin in a week. What are you doing.
Players: We start digging our way out.
3e: DM does some math, and either has them start making rolls, or computes an average, and figures out how long it will take them to get through the wall. There's nothing to stop them or harrass them, so they just go at it.
4e: Pretty much the same, except the DM can just decide how long he wants it to take for dramatic purposes, or based on past experiences with tunneling through rock. He can say, "Oh well, you have a dwarf with you, so he lends his expertise and you make relatively good time. It takes you until late afternoon to safely tunnel through to the passage beyond." Or whatever he would like to make it seem exciting or tedious as fits his needs.
#3
Same as #2, except the DM knows that the noise they make tunneling will attract the boojum, which will burrow in and attack.
3e: They start attacking hardness and hit points, the boojum shows up long before they get out and attacks. They kill it, and take it's tunnel out or finish theirs.
4e: See above.
Basically, the designers decided for 4e to focus on dramatic moments. Combat needs good, consistent rules so that it is smooth, engaging and easy to master (rules-wise). Breaking through a wall doesn't because it's always either a dramatic moment (in which case, you don't have time) or it's a non-dramatic moment (in which, it doesn't matter).