The exits set to no overwrite should be easy to fix, so never fear!

The reason I put in time limits for almost every level is because that IS the way Lemmings is meant to be played. If you look at the official lemmings games, they all have time limits. There are exactly 2 levels in SubLems that give unlimited time: Sweet 5 and Chaotic 15. The reason Sweet 5 has unlimited time is because, it was initially a mistake that I forgot to give a time limit. I then realized it fit the theme of the level "A lemming party NEVER STOPS!" Chaotic 15 has unlimited time because it is an EXTREMELY long level. I originally gave 10 minutes, and that was a time crunch. I changed it to avoid aggravating players. I personally like time limits. They force you to be quick as well as smart, which are two important skillsets to have in Lemmings, and time limits can help you determine how long a level will take.
I will make sure to make the traps more visible in the next update, with the exception of retractable boulders from the dirt tileset, since that is kind of their point, and I only use them secretly on one level, which is EXTREMELY short, so no harm done.
As far as long waiting periods go, I don't mind them at all. I find those help me make more detailed levels and let people appreciate the music. Also, most of the levels with long periods of just walking were some of my first levels; I tend to avoid those long spaces of nothing now.
As far as individual levels go:
Newbie 1: I thought that was the perfect way to start the game, but I understand that the release rate is not always intuitive. I'll try to find a way to fix that.
Newbie 5: One of my favorite level designs! I'll try to make the gaps more obvious without making Block-O-Tron look broken.
Newbie 10: the only level that requires the nuke. I thought that was just a fun way to end the difficulty. It also teaches people the importance of waiting in lemmings.
Sweet 3: I agree, that level is pretty long, but it really is as boring as you make it. You can alternate upper and lower routes if you like. This was also the first level I made for SubLems!
Sweet 4: It's a rip-off, yes! But I thought recreating one of my favorite levels from Fun in winter theme was cool (pun intended). This admittedly could be in Newbie.
Sweet 6 and 7: good to know these were a bit of a difficulty spike. I'll probably move these back.
Sweet 9: Agreed, this level is a bit too large, but part of that is 1: it is one of the first levels I made and 2: it gets repeated later, and more space is used.
Sweet 10: this was the hardest? Really? Interesting, I almost put this earlier.
Sweet 11: I gave 165 minutes because that is the length of the play Much Ado about Nothing.
Sweet 12: I originally made a harder version for Hellish that used more of the space, but that didn't work out, but I kept the Sweet version as it is.
Sweet 13 and 17: these levels are as fun as you make them, and smart players can solve them easily, so I'll leave the backroutes in. They're boring, but reward players for their intelligence.
Sweet 15: better fix the noose. This level ALMOST got the boot from the pack, but I kept it in for nostalgia. If I can come up with better level ideas, this one and its sequel may get the boot in an update.
Sweet 16: this would be the hardest level ever in DOS Lemmings, but I think it is a good intro to directional selection in SuperLemmini. This is not in Newbie because it is less intuitive than the release rate.
Sweet 18: I call it "masterpiece" because of the architecture. I think it may be the prettiest level in the pack, but that's just me.
Sweet 19: I made the gap so big because I wanted it to be a true "abyss". If this bothers people, I can make it smaller.
Sweet 20: the traps are invisible because I wanted you to have to "kill" the boss by bashing through him, but making the traps visible made the boss look stupid. It's a short level, so I think they're all right.
Hellish 1: like Icho, you made that level MUCH HARDER than you needed to.
Hellish 2: if you perform the trick from the opposite side, then the ground is marked where you should put the digger. I'll try to mark the other side as well.
Hellish 5: the only reason (and I do mean the ONLY reason) the level is so big is because it would look awful if only the crucial parts were in. Seriously, it looked TERRIBLE.
Hellish 6: never fear! I will reduce the amount of building necessary for this level.
Hellish 7: I made this level LONG before I realized it was a jaded solution. Sorry! At least the music is awesome...
Hellish 8: backrouted HEAVILY.

Going to have to fix that AND Icho's backroute...
Hellish 9: I will significantly reduce the number of "jumps" needed in the update.
Hellish 10: there's an idea. I think I will shorten the gap between the ice cream dishes. Thanks for the suggestion!

Hellish 11: one of my favorites as well! I feel if I made the ledge longer, the level would be too easy for the Hellish rating. Hellish is supposed to be REALLY hard. I may change that though...
Hellish 12: indeed, backrouted something AWFUL! Easy enough fix though...
Hellish 13: I thought for sure I found all the backroutes to that level! Darn it!

Hellish 14: HOW DID I NOT SEE THAT BACKROUTE!? I don't think it will be possible to fix that one...
Hellish 15: this is one of the main reasons I can't convert to NeoLemmix; it gets rid of awesome tricks like this!
Hellish 16: slightly different beginning than I planned, but same solution overall. The long walks actually WERE intended for this level, as the point is these Lemmings are marching for the galactic empire. Just imagine them marching to the Imperial March.
Hellish 17: backrouted slightly, but I may leave that in. It is still really hard to figure out and difficult to execute.
I didn't even check Hellish 18 because Icho discovered a GIGANTIC backroute. I too am really puzzled by the strange brick mechanics. There are no empty pixels; it is solid, but lemmings still treat it like empty space. I'll try to fix that as well.
Hellish 19: backrouted on one of the hardest levels in the game. Shouldn't be hard to fix. You were not supposed to build over the basher tunnel entrance.
Hellish 20: don't worry, I'm halving the amount of lemmings you need to solve and making traps more visible!