My problem with SMB3 was its level design. Sure the levels had variety, but variety very rarely existed in the same level. Each level had it's own gimmick then it moved on. SMW is a much more complete and complex game in terms of level design.
This is true, but I think you're taking a positive as a negative. I DO enjoy the longer levels of World, but at the same time the fact that 3 kept everything tight and focused helped to make progressing feel fresh; you only spend three or four minutes on each gimmick, after which you are rewarded with a completely different gimmick. This helps with the pacing, as it means that I get a few minutes of creativity, and then I usually get to try something completely different. This isn't the case with World: because many of the levels share the same tricks, the levels start to feel too similar for my tastes.
If I had to come up with an analogy, I'd say that 3 is akin to a buffet where you can take small samples of a ton of dishes, while World is closer to ordering the T-Bone Steak; tasty, but once you dig in things aren't going to change much.
One shortcoming of most Mario games is that exploration is often rewarded merely with coins and one-ups; as a kid I really derived great satisfaction from discovering a second gate by flying under the first gate on Cheese Bridge. Star Road and the Special World really buck the trend in Mario level progression.
I agree that finding an entirely new level is probably the best way to reward exploration, but I think you're selling the other Mario games short. In the first one, exploring netted you new lives, which in that game were otherwise as rare as splinters from the True Cross: they meant more than they do in any other Mario game. There were also plenty of warps, which can't be understated. Mario 2 often gave you more lives or health, as well as level warps of its own. Since 2 is a pretty challenging game, all of those were more helpful than World's rewards.
Mario 3 had extra lives buried in nooks and crannies, which admittedly were less valuable than in the original, but it still took a skilled player to keep their life count in the double-digits. Exploration was also how you found those rare suits, flutes, clouds, and other power-ups that made progressing further that much easier.
I think you're getting spoiled by World's extra-life system, where you get a 1-up for walking at least ten feet, to remember just how precious a reward that green mushroom could be. While I actually prefer that extra lives be plentiful, and I kind of hope that future Mario games altogether abandon the pretense that running out of lives means anything, the rewards in the NES Marios were very well tailored for their respective games.
Having said all that, I did like exploring more in World than in 1 or 3 (but still less than in 2).