Recently gamers have been receiving a lot of inside information and insight into the development of games such as The Legend of Zelda and Super Mario World, mostly due to the recent Zelda release this year, the upcoming Mario game due out this month, and the release of the SNES Classic Edition. One tidbit of neat news centers around how Yoshi became a playable character in the Super Mario franchise. The reason is that they wanted to mix up the gameplay for a 2D title and make it feel fresh by adding new mechanics only Yoshi could use.
That's exactly how Yoshi's Island plays out: Yoshi carrying Mario to the goal.
The interesting thing about this particular development was that it was a huge departure from the original SNES version of Super Mario World. For one, you were no longer controlling Mario or Luigi. Instead, both the Mario brothers were babies in Super Mario World 2, effectively making it a prequel to the previous game.
Additionally, the controls had changed quite a bit in the sequel, including Yoshi being able to suck things up and turn them into eggs. There was also the added feature running in the air, shooting eggs, and collecting the flowers to increase the timer for when Yoshi would get hit and Mario would fly off his back in a bubble. In fact, the whole bubble gimmick was one of the biggest changes in the game compared to previous Super Mario outings.
In the old games, Mario would effectively get up to three hit points: one when he was little Mario, another when he gets the mushroom, and a third if he managed to get a power-up. In Yoshi's Island games the health points were handled differently, with Yoshi being able to take infinite amounts of damage, but Mario and Luigi having a limited amount of time to be retrieved once they started floating away in the air in a bubble. Players would have to race to their location, pop the bubble and get them back on Yoshi's back.
In the previous game Yoshi was played more like a tool, but in the sequel Yoshi was played more like a guardian.
According to Takashi Tezuka, this was a purposefully done mechanic because the team wanted to design a game that could help lure in and introduce newcomers to the platforming genre. The learning curve was a lot lower and a lot more friendly for those who were just starting out. It's a fascinating read for anyone who was remotely interested in the design methodology behind the older Super Mario titles.