Okay, I played this, and I kind of apologise for not posting here earlier.
It's a good hack in general, with all kinds of interesting custom sprites, blocks, graphics and music, but the level design does need a bit of work (however hypocritical it makes me sound to be saying this).
But now to each level individually.
First of all, the space level with the planet/Earth/whatever from Seiken Densetsu 3 in the background. It's all very much like SMW YEAHH, and looks like that level on the moon in said Japanese hack. Okay, the background is far better, but the concept is similar, and the use of the Koopa Elites just promotes said similarity more. This is a good thing by the way, I'd just have found it cooler if you'd taken the binary number glowing platforms and put them in too.
http://www.youtube.com/watch?v=0bq_3eGI4Wc
^Yeah, those platforms would have gone well.
The church level looks awesome. I can tell you were inspired by Toad's World too, I think the level music and name were the same. Talking of which, what is the music here? It's pretty cool, and I'd like to use it.
But yes, the background is truly amazing. I love the cartoon character stain glass windows and rows of candles in the background, it makes it look so... atmospheric. Not sure about the Yoshi's Island foreground, it just seems to blend in a bit much.
I liked the level with the mice...
The castle level had okay use of the Ardy Lighfoot graphics, and had the SMB 3 Bowser sprite, which seemed fantastic. However, it was a bit flat and linear... and believe me, considering the type of level design I don't consider linear, that's not such a good thing. It felt at times a bit 'run right, dodge custom sprites, maybe jump a bottomless pit' as far as level design went. Maybe some more elevation, more tricky platforming, more level design in general here?
All in all, the other levels I can't remember much. I know one level used Tales of Elementia graphics, another used Zingers with Mega Man graphics and there was obvious a SMB 3 styled pipe maze, but while it all looked and sounded fancy, and the sprites and bosses were amazing, it felt a bit... flat level design wise. Bit like my opinion of Brutal Mario really.
Still, I liked it. It's almost like a Japanese hack really, the kind you'd see over at this site (http://www7.atpages.jp/smw/view.cgi) or the VIP 5 file uploader. Indeed, if this wasn't on SMW Central's hacks to be moderated list, I'd think this was a Japanese hack by some individual over there, like what Puya (Scarlet Devil Mario) or EXTRA (Mario LD) or Puriny Ping (The Mario) would make. I like how you've used all the fancy resources I uploaded recently after finding on various Japanese hacking sites, and if you ever want some more sprites in future, I've recently added about fifty I found on my computer that could be used here.
The overworld could do with some work... again, if you want custom overworld ExGFX, I'll be happy to add some to your hack to make it look nicer. Just say what game's overworld graphics you like/want used and I can send you the ExGFX and add them to the ROM.
Edit: Some more thoughts, mainly on bosses:
1. Your Bowser boss doesn't work completely, the door afterwards doesn't open or lead anywhere, making the level unbeatable. Once I beat it with a code I was off to the final level though.
2. Finally, someone uses my Yellow Pix sprite!
3. Replace the Thwomp boss with either Dahnamics's version or Yoshicookizeus's 2.0 version. I can send you the latter, I think he deleted the folder with from his files.
4. That final boss took forever, and the fact the Newbie boss was smaller than the graphics meant many shots go right through it.
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