I can't speak for the community but as for myself, 'wow' is something that is original. eg.
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First part of a three part look at some of the shining examples of uniqueness.
Mods:
SRB2Riders - original because it is Riders style levels and such made for SRB2, and to add to that, they optimized the exe for the Riders gameplay.
SRB2JTE - Bots, enough said.
Acid-Missle - Uniqueness in level form and particularly bosses.
Blue Heaven - The finished product promises to be an alternate SRB2, also with a far more forgiving open air level than you would think, for you AGZ players that just can't get past the small platforms.
MLP - Very original level designs, from the moving floors sections beyond, it proves that two levels of the same precise theme can still be radically different. Also has some of the best bosses I've seen.
Chaotic Darkness - Very promising, and the emerald stages can only be rivaled by MR.
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A special area for extraordinary levels that are enough on there own to give me a WOW factor with just one level.
Levels:
Metalflower: Takes everything one could hope for a factory level to have, and makes it one big level. With the potential of becoming a full zone, this level is amazing and difficult.
Egg Palace: A subterranean counterpart to Areal garden with interesting surprises.
Moon Temple: Gimmick after Gimmick after Gimmick, and with the familiar AGZ style we know and hate.
Nitric Citadel: (see below)
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Now for a large section analyzing the biggest mod in SRB2 history as well as probably the most lauded level pack on the site.
Mystic Realm - A complete mod with seven full zones, one secret zone, and probably the only decent Supersonic stage here to date. Add that to the secrets, including playing an alternate version of the main game with different themes and more difficult enemies. The wow factor is most apparent on the diversity of the levels. I will give a list of each of the unique levels with a description to give you an idea of the diversity.
Beach (yeah thats original, *roll* but a Sonic game is incomplete without one) - easy and forgiving, but lots of paths and secrets.
Valley with Slime - Its your average level with pools of harmful liquid. However there are no factories. Alot of it remains to be simple platforming, but now has a penalty for messing up
Verdant Forest - Very seldom see levels like these, it seems mazelike, but you never get lost. (unless you are as stupid as I can sometimes be) It has alot of navigation with some open air platforming in the second act
Outdoor Volcano - Yeah we all know and love indoor volcano levels, but this is just on the outside. It is like a lava cave level with the advantage of having lots of open air segments that seem far less confined.
Ice - I think this started the trend of harmful ice water, but the use of the ice is very clever. Each level is short, but at the end is a grand finale that will make Sonic rue the cold day he came there.
Water and Factory? - Odd combination, but it works so well. It may not be as exciting, but it really gives away that flooded feel. It has underwater level features, with added factory obstacles. It also has what I consider the best emerald stage in the game but more of that later.
Open-air - Love-hate levels, everything is over a pit. It focuses on platforming, the catch? Dodge hordes of Jetty-syn's at the same time, as well as avoiding the red crawlas waiting for you at your destination. Each level has a few central areas, with the second giving you two paths each showcasing a gimmick. One has constantly moving platforms (my personal favorite) while the other has what look like floating logs, platforms that must be jumped at just the right angle or it would be almost impossible to land on the short side. (white shield for the win!)
AGZ Act 3 - Linear but gives you a chance to take each gimmick on its own in any order. Blue springs, floating logs, moving platforms, Jetty-syn hordes.
PAZ - The same thing that makes many hate it so much is what makes it special, it has difficult platforming, but no harassing enemies. Of course, being the cruel mapper Mystic is, he adds a feature that makes it still wicked hard. All sectors have 0 height. (well almost all) This means it is harder to calculate the distance. I do carry the opinion however that Mystic should split PAZ1 up into 2 as well.
Mystic Realm - Super Sonic stage that is not too boring but also keeps you occupied. Rings are hard to get to and take time. However they are necessary for you to reach the boss with sufficient rings remaining to fight him. o_O This level is THE hardest SS stage I've played. (ofc I did skip the 4 zones preceding it...)
Emerald Stages: Each are similar to the level's style but have a new feature that make them extraordinary.
Mudhole Karst: Similar simple platforming, but now has damaging mud, and is much more mazelike.
Toxic Citadel (?): Keeps the slime theme and then sends you into a castle with many crushers.
Labyrinth Woods: This level is impossible to go through by mere guess, makes you wish you had your compass...
Vulcan Forge: Lava factory, I love these levels! Pipe balancing and lava make for an interesting mix. Add crushers, cc's, and classic factory platforming and you get a winner.
Silver Cavern: What is harder than slipping? Slipping on platforms over pits. From climbing vast corridors to navigating dark caves with foreboding pits, it is sure to please those wanting a challenge and prepare some for the larger challenge to come in the form of AGZ
Nitric Citadel: My all-time favorite level, I wish it had a second act! You've got your factory and your turrets and the slime/acid. The catch, acid in insta-kill. With fixed and popup turrets harrassing you, while you rush to avoid the acid or squeeze out of a tight situation. When you are near the end, you are rewarded for your hard work with THZ sky with far more forgiving platforming.
Starlight Palace: Open air again, but this time, two paths take you on two different routes through the same central area. Couple that with the illusion of horizontal moving bridges near the end, makes it an interesting level. (oh and you can climb now)