Preliminary note: What follows is my opinion as someone who has followed the development of NeoLemmix and played a little, but not yet seriously played a NL pack with the intention of solving for myself, nor designed any levels for NL. However, I have in mind that I would love to play and design NL content when I have more time and when the game and its features are more stable.
Original Lemmings presents a series of challenges to the player, within the framework of an unchanging set of rules. There is also the challenge of figuring out what the rules are, but the player is helped through this by the first seven levels, often referred to as the tutorial levels even though they are not explicitly designated as such. However, there is one gimmick level where the
nature of the challenge is changed -- Tricky 28, Lost Something. Perhaps also We All Fall Down, Bomboozal, and Just a Minute could be referred to as gimmick levels, as what you have to do is substantially different from the rest of the game. (ONML, of course, has the Superlemming gimmick...)
These levels have not aged well. Lost Something is pretty pointless once you know where the exit is. We All Fall Down is probably the most disliked level (series) in the game. Bomber timing as in Bomboozal is so unpopular that Lix and NL no longer support it. Inroducing Superlemming is actually a pretty decent puzzle, but it would be better without the gimmick.
That's not to say that all gimmicks are necessarily bad. In DROD, playing as a monster may be considered a gimmick. (In standard DROD, you control a human swordsman who must kill all monsters with his sword. Some levels have you controlling a monster, who can kill other monsters by stepping, but as the game is turn-based you must avoid stepping onto one monster such that you become vulnerable to another.) While this gimmick is used in a small minority of the total of DROD levels, it is very popular and many excellent puzzles have been created using it.
However, the disadvantage of any gimmick is that you have to learn a new set of rules. And indeed, some gimmicks have no point except the one-off joke of making the player figure out that a new rule is in effect -- I don't know all the gimmicks by name, but the clearest example is that one where you can continue assigning skills after the skillbar displays "0".
NL (currently) has too many gimmicks: there are too many rules for a new player to learn, for not nearly enough reward in terms of interesting puzzles. Some of them were okay as one-off jokes, and I wouldn't mind if the gimmick were culled from the editor + future levels, but kept in the pack where it originally appeared. (The first pack to use gimmicks did use a special music to warn the player that
some new rule was in effect.) Many of the existing gimmicks are just not useful in puzzle creation, and I'd be happy to see them go. A couple, such as "No gravity" and "Unalterable terrain", at least enable puzzle concepts that are not possible without the gimmick -- but I just don't feel they add enough to be worth the annoyance of dealing with extra rules (and these two in particular tend to lead to very fiddly levels, from what I've seen).
So, rather than list every gimmick (as I don't know them all), I'll just stick to a list of ones I would like to see kept, and why.
Karoshi -- this allows puzzle concepts that are interestingly different from standard levels but equally fair, once you know how the gimmick behaves. The gimmick is discoverable as the preview text says "Killed" rather than "Saved".
Rising Water -- this is easily discoverable, allows new puzzle concepts, and should not be too annoying. It could be a little annoying as there is one more thing you have to estimate, in terms of balance against the timing of lemmings' walking speed, but I think it could be used well. I would like to use it occasionally.
Zombie/Ghost -- I haven't studied the game/editor enough to know why these are listed as gimmicks. If you just mean retaining the zombie/ghost game elements, then they certainly lead to new interesting puzzle concepts (I have seen them used well in namida's videos of mobius's levels) and are visibly different from normal lemmings.
...and yeah, that's about it
In response to mobius, I don't think
One skill per lemming need be kept as a gimmick, since it exists as a possible setting for talismans.
* * *
Secret levels are a very different issue. The way they were originally implemented was annoying: you had to find an invisible trigger, which could be anywhere on the level. Hitting a secret level trigger by accident could mess up an otherwise good solution.
However, I seem to remember secret levels can now also be unlocked by getting a talisman. I think that's a nice feature to have as a possibility. Another is unlocking secret levels by completing every level in a rank: then you could have unlock-all and still have a reward for completion. DROD has secret rooms and levels. (A DROD room is one puzzle; a level is a set of themed puzzles; a level is like a rank in Lemmings, except that a typical hold has more levels and fewer rooms per level). These were first implemented in
Journey to Rooted Hold, and in that game, the typical method of finding secrets was noticing cracks in the wall, indicating a wall you could break with your sword. The usermade hold
Gigantic Jewel Lost was greatly innovative with secrets whose access was clearly visible, but required clearing an outer room in a more challenging way, and this idea was incorporated into the latest official hold,
The Second Sky. It has proven immensely popular with the DROD community, though as it's quite difficult to make a dual-solution outer room, there are still many secrets hidden the older way.
One reason for the continued existence of secrets in DROD is that there is usually a "master wall", which opens when all secrets are found, and unlocks the final layer of content. However, even without this, the excitement of finding secrets has always added to the pleasure of playing a well-designed DROD hold.
