Demanding Dragon is definitely a card that casual players can get very excited about. When he enters the battlefield, he deals 5 damage to target opponent unless he or she sacrifices a creature. That's pretty powerful, either way. Plus, he's 5 power and 5 toughness. While he may be no Thundermaw Hellkite or Stormbreath Dragon, forcing an opponent to sacrifice a creature is always a powerful play. Five damage is no joke, either.
As powerful as he sounds, why is he only $1? Well, this isn't a card that screams competitive play. But this isn't just a Dragon. This is a creature with a generically powerful effect that can slot into a variety of midrange decks. Really, his strength is very dependent on the meta. If there are decks that can effectively swarm the board, he's going to be a fairly weak play. But, he's really strong against decks that play one big finisher or don't play many creatures at all. So, there is room for Demanding Dragon to be very good.
In Standard, Demanding Dragon felt more like a sideboard card against particular decks. M19 is full of cards built to hose certain decks and this is definitely one of those. There are definitely ways to abuse his enter the battlefield ability, such as blinking and reanimating. If those types of decks are viable in Standard, he'd definitely be a player. Also, the recent best Dragon in Standard, Glorybringer, wouldn't hang around past October 2018.
We're not expecting a huge price tag on this guy, especially being only a rare. But, since there is so much interest in him, you should be keeping an eye on him. Attaching a Lava Axe to a creature that can at the very least force your opponent to sacrifice something or take a sizeable amount of damage to the face is awesome. Yeah, your opponent can choose not to sacrifice something, but losing a quarter of your starting life total is pretty sizeable.
This should be an interesting card even without a ton of Standard play. 5-mana Dragons typically do see play. He doesn't have haste, which hurts him a bit, but his abilities make up for that. It seemed like this could be a $3-4 card very easily, especially with his demand profile early on. It never happened, though. With that said, he is a Dragon, and his foils bottomed out at $2. So, there is some value in foil Demanding Dragon. He's not going to make you rich, but buying at the dip isn't a bad plan if you plan on using him in some mono-Red EDH deck.