It's difficult to compare. Besides attempting to ignore rose colored glasses, I admit my sensibilities are different now. They've evolved or changed entirely. Additionally, without any proper documentation of my feelings when playing those old games, I'm strictly going by memory. I wonder how many people remember 20 year old events, the sights and sounds of it all, accurately.
For me, the fairest assessment of older titles is whether they still hold up. Most Mario platformers do. Yoshi's Island doesn't. Not that it's a bad game, but I remember borrowing it from my neighbor and beating it. Maybe 10 to 12 years later, I went on a retro gaming binge and tried to find a copy. When I finally ended up playing the game again through the 3DS Ambassador Program, I didn't really know what I saw in the game. The graphics were still charming, but the gameplay didn't grab me.
Donkey Kong Country didn't hold up well. At all. Every time I've played the game years removed from its original release, it felt generic, almost boring. I feel like I was more enamored by the graphics which were Holy-****-Snacks in 1994, but they're not amazing anymore. They don't even have that old school charm. Pixel sprite graphics hold up better than the rendered sprites do in a retro-chic kind of way. With so many gaming years under my belt, I just can't be taken in by graphics anymore. I still appreciate them, but I need more to justify the time spent with a game. Donkey Kong Country Returns mops the floor with Donkey Kong Country. It's far more inventive and polished. The graphics are nice albeit not groundbreaking, but I feel like they'll hold up better because Retro Studio's art design is so far ahead of Rare's.
I feel as if the entire New Super Mario Bros. series will follow Yoshi's Island's lead: a game I enjoyed at the time of release and while they may never be bad games per se, they lack the timelessness of the original Bros. series and World.
I think modern 2D platformers have the potential to be better so long as developers use the past to make their games better. That's what makes something like Donkey Kong Country Returns great and other games merely great at the time. I know this isn't exclusively about Nintendo's 2D platformers, but they're really the only ones I've been exposed to. Besides a Game Gear I rarely used, I only had Nintendo hardware until the PS One in 2000 so I played very few non-Nintendo 2D platformers for an extended period of time.
My hope for future first-party non-Retro Studios Nintendo platformers is that they learn from the past and apply it to new games. The New Super Mario Bros. games lack charm and innovation. They feel like they're made using a stage editor. Such games can still be fun even if they feel derivative. What made certain old games so memorable and legendary? That's how to make a game better than the old ones. It's not about copying the successes of those games or pulling on the strings of nostalgia. It's about getting in that mindset and coming up with inventive gameplay mechanics. Super Mario 3D Land isn't a 2D platformer (though it has the same feel of one), but it's memorable for the same reasons why the New Super Mario Bros. series will not be. It effectively uses past games to create a fun experience.