After some consideration, I feel that the best answer is to simply remove all of them; even including Superlemming. While some may be quite fun, I do think they go against the increasingly-strong desire for consistency.
NeoLemmix V1.43n-B (or whatever the last version that supports them ends up being) will remain available even after newer versions get released, to ensure that a version that supports gimmicks is available for those who want to play old levels that use them. In particular, it willl be important for converted-from-Cheapo levels, since NeoLemmix conversions are currently the optimal way to play Cheapo-origin levels on modern PCs where Cheapo itself doesn't work very nicely.
Wrap-around might eventually see reintroduction as a proper feature, most likely implemented in a fashion more similar to how Lix handles it than how NeoLemmix currently does. In regards to deadly sides, I would rather not have the behaviour of the sides differ between levels; I am open to changing the behaviour in general but would like one common behaviour across all levels. There is a seperate topic for discussing this:
http://www.lemmingsforums.net/index.php?topic=2537.0In regards to secret levels, it was raised on IRC several days ago that while secret level triggers are annoying, secret levels themself aren't nessecerially so bad if unlocked via less-annoying means, including the current alternative option of unlocking them by obtaining talismans. I am almost certianly going to remove secret level triggers, but I'd like further discussion on whether these other methods of reaching secret levels should be kept, or if the secret level concept should be ditched altogether.
Once I'm finished removing the code for gimmicks (since that'll tidy things up a lot) - and probably doing some other general tidy-up of physics code (the remove-commented-out-or-never-used-code type, not the actually-change-anything type) - I'll probably create topics for every action in a similar vein to the current topic on Climbers. This way, we can both discuss if there are changes that need to be made, and potentially spot bugs more easily. At the moment, I've made a version that's gimmick-free, but the code to actually implement the effects of gimmicks is still mostly there (apart from Superlemming and Bait-and-Switch; it also doesn't display the modified preview/postview screens for Karoshi, but the in-game code for it is still there), LemGame.pas's "TLemmingGame.CheckGimmick" function (used to check if a level has a certain gimmick on it) just always returns FALSE. Actually stripping out the code that implements them is the next step.