Down B is a really great move.
I often walk away and down B if the enemy wants to follow me on the ground. If you did it a few times (or once) the enemy will probably try to appraoch from the air the next time, so be ready to bair them if you did this.
Also, if you use down B on a platform on battlefield right above your opponent and they stand on the ground you can hit many characters. I didn't test every character for the usmash and crouch things, only the ones noteworthy.
For most characters it's bad to stay on a platform above their opponent, but not for DK! (well, watch out for aerials though, this only works if the enemy is on the ground)
You can hit those characters (in their standing animation):
ROB, King Dedede, Falco, Samus, ZSS, Snake, Bowser, Ganondorf, Marth, Ike, Sonic, Charizard, Captain Falcon, Peach, Luigi, Wario, Pit, Zelda, Shiek, DK, Diddy, Mario, Link, Fox, Ness, Lucas, Toon Link, Ivysaur (you have to be right above him otherwise it doesn't work, and it's risky because of his neutral B and usmash)
Characters that get only hit on a certain part of their standing animation (it's not really reliable... you shouldn't use it on those):
Jigglypuff, Wolf, Squirtle, Pikachu, Ice Climbers, Mr. Game&Watch
The characters you can't hit:
Kirby, Olimar, Meta Knight
You can also hit during their crouch:
King Dedede, DK, Bowser, Ganondorf, ... (didn't test everyone)
And you can even hit these during the charge of their usmash:
Bowser, Zelda/Shiek, ROB, Marth, ... (didn't test everyone)
It also beats those usmashes:
Wolf, Ice Climbers, Ike, Yoshi (not always, for safety don't use it against him, unless you're too far away for his head to hit you), ... (didn't test everyone)
You can hit those characters if they are using usmash (and they can't hit you):
Fox, Toon Link, ... (didn't test everyone)
Keep in mind I only tested it on the 2 lower platforms on Battlefield and that the height of platforms from other stages might not be the same (smashvills platform is a little higher).
On smashvilles platform you can also hit Dedede while standing, but not if he crouches. But it beats his utilt! If he doesn't DI left or right and is at 0-5 % he'll be hit onto the platform and hit again it you still use down B.
Btw, if DK runs at an edge and uses down B right before he leaves the ground and you hold the controlstick forward DK will start his down B animation and cancel it and go off.
I tried so much and so long to get the first hitbox out and cancel it, but it never worked. I tried it in slow motion but it seems that DK can't go off after a certain part of the animation... but it works if you have the bunny ears. Maybe there's some way to get a longer slide than from running? :/ I tried craq walking, but the boost isn't as much as if you run.