Yoshi's Island is a throwback to Nintendo's NES days, when they would release an utterly different and experimental sequel to their biggest hits with no fear whatsoever. Though it bears the "Super Mario World 2" name, Yoshi is now the main character, toting a baby Mario on his back ... well, to be accurate, it's actually a series of Yoshis who each take on one level as they relay the baby around their island.
And though it shares the same baseline "cutesy platformer" ground with the rest of the mainline series, the level design and mechanics are different. There's more of a bent toward slow-paced exploration of mazelike levels rather than twitch platforming, and Yoshi handles differently than Mario and with several new abilities. He's pretty comparable to Yoshi in the original Mario World games, but a little heavier and less agile. He can also chuck eggs, ground pound and flutter in the air, all abilities that weren't seen in the previous game. The art direction is also suddenly all about pastels and watercolor (somewhat similar to Kirby 3), and though Koji Kondo returns as composer he opts for a very unusual combination of carnival music and jazz fusion (which comes off WAY better than that description would initially lead you to believe).
This is purely my own speculation, but I think the radical new direction of this one was influenced by a couple of things. One is that the game was released late in the SNES's life, and during a period of general anti-platformer sentiment when polygonal 3D on the brand-new Playstation was the new hotness. Bubsy and his ilk had also just spent a few years poisoning the platformer well. So Nintendo wanted to go in a very different, even shocking direction with their flagship series. The other factor I think influenced this one was a desire to differentiate from the Sonic series, which had proven to be very stiff competition in the platformer arena. Sonic was all about finding the optimal path through a level and blasting through it, so Nintendo opted to slow things down and center them more on exploration and collection instead - something that Sonic was very ill-suited to with his herky-jerky movement and floaty jumping when not Gotta Go Fasting.
The blueprint for this transition to a more exploration-based, collect-a-thon style had already been laid by the Super Mario Land series on the Game Boy, particularly the second entry on (where Wario started to get involved). You can see Nintendo doing here what they did with the Mario Land games; let a supporting character take over lead character duties for one installment, before creating a separate spinoff branch for them.
This "collect-a-thon" style actually prefigured the dominant way that 3D platformers would work. Here in the 2D space, it doesn't come off quite as well. You get a score shoved in your face at the end of each level based on how many of the optional flowers, red coins and stars you collect as you pass through each level. In spite of the new focus on exploration, the game proceeds on a linear track, with no secret exits from levels to take you off the main branch; the only way to access the two hidden levels in each world is to post a score of 90 or better in each of the main levels. There's little purpose to doing anything optional, however - no shortcuts as in other games, and just playing the regular levels usually plies you with such a pile of extra lives you'll be well into the double digits all the way to the end of the game.
Attempting to suss out all the coins/flowers/stars in each level is pure madness, thanks to the more iffy-than-usual play control and level design. Everything is adequate to get through the main level, a few points of frustration aside. The game can be maddening on the occasions it decides to dip into old-school Mario precision jumping bits, since Yoshi isn't built for that; off-screen cheap hits and enemies appearing out of nowhere don't help. It's also irritiating on occasion when a cave or castle gets a little too mazelike and backtrack-y for its own good. I still wouldn't call the game "difficult" when simply following the main track and getting through levels as efficiently as possible, but trying to get to all the optional stuff usually involves some incredibly frustrating sequences.
Another thing that has to be discussed is the shrieking. Yoshi is nearly invincible in this one, dying only when dropped down a pit / into lava, or on a small handful of environmental bits that are instantly lethal (like spikes and thorns). However, any hit propels Baby Mario off your back and to the skies, where he floats in a bubble and cries his little head off with a repeated squeal that rivals Navi for annoyingness. The game often seems to float him intentionally out of reach for a few seconds just to squeeze a few more WAA WAA WAAs in, but what's even worse is that it not infrequently shoots him somewhere behind enemies or lethal obstacles where you can't get to him.
Big fans of the game try to handwave the crying as "well, he's letting you know he's in trouble, you want to save him to shut him up rite", but that's a weak argument. The same thing could be done just as effectively with a much less annoying noise, and the recovery routine definitely needed tweaking so that it isn't so easy for him to blow out of your reach for such long periods (reducing the length of Yoshi's somewhat ridiculous stun-lock animations would also help). The obnoxious noise represents a punishment for every single little slip-up, not necessarily a fatal one, but one that chips away at your desire to continue playing the game the more you have to hear it. You don't see any other Mario game busting your balls like this for getting hit while you have the Super Mushroom, you just shrink down and go on about your business.
Mario's cry may make you feel like the game is trolling you, but that isn't the only thing. The whole level design in general feels very trollish at points. It's hardly I Wanna Be The Guy
, and still not really a difficult game ... but it feels closer to that sort of thing (or Mario save/restore ROM hack territory) than any of the other mainline Mario games I've ever played. You got enemies appearing completely out of nowhere, hiding behind opaque background objects, and projectile-firing guys taking aim at you from off-screen before you even know they are there. The game also uses a Super FX 2 graphics chip, and they apparently decided to justify the cost by adding a bunch of giant scaling cheap hit traps. One of the level designer's absolute favorite tricks is to dangle one of the collectible flowers in a seemingly innocuous place, then spring some sort of unpredictable background trap on you when you go for it.
So the overall theme and feel is very "trial and error", or having to know what's coming to successfully navigate a level and get all the goodies, more so than you get with the other mainline Mario games. The emphasis on graphics and big, flashy effects (gameplay considerations be damned) makes it feel more like a Treasure game sometimes than something by Shiggy.
In the original Super Mario World, I wanted to 100% the map and find all the secret exits, just for the fun of it. This is my third run at SMW2 since the '90s now, and every time it's been the same - I just don't give a shit about the collectibles or hidden levels at all, and I don't ever want to play levels again once they're behind me.
I've also never actually finished this game. This time out it lost me in the 4th world somewhere, in one of the castles where you were expected to go roaming back a million miles to find eggs to break shit, passing through a maze of tedious garbage and potential cheap hits / respawning enemies. Fuck off with all that, game.
At the very least, you're assured some of the best eye candy on the SNES. Nintendo had full mastery of the system hardware at this point and was showin' out on their way out of the 16-bit era. But you often feel more like you're hanging in there for the next visual treat rather than playing for the sake of the gameplay.
Whether you like SMW2 or not, it's a fact that this is very experimental for a mainline Mario game. I feel like the experiments here don't add up to an improvement on the original formula. This approach is very uncharacteristic for Nintendo, and I feel like it was only done because the SNES was almost dead and they had already commited to the idea of basically rebooting the series with each new console generation. SMW2 is undeniably polished and frequently enjoyable, enough to get a 4/5 from me, but I don't think it belongs among either the best of the Mario games or even the best of the SNES games. The visuals alone are probably good enough to make it worth a run, but don't be too surprised if you find yourself impatient for it all to just wrap up.