Time limits are an interesting one. Personally I'm quite a fan of 1-minute and 2-minute levels, as they can give a bit of an "oh hell" or "that's impossible!" feeling. For example, seeing the 2-minute time limit on "The Fast Food Kitchen" (it's 3 minutes on the DOS and Windows versions) really made me think "there's no way!", but it is perfectly doable if you do both sides at the same time, and it becomes more of a multi-tasking challenge than a time crunch, but you actually NEED the time limit to enforce the multi-tasking.
Bad examples of tight time limits are where a long and convoluted solution is made even harder by only allowing a few seconds of leeway, which can be unfair if a failure occurs simply because you didn't blow up the blocker sooner, and also runs the risk of a porting failure because the exact time limits vary between versions (you lose about 3 seconds waiting for the trapdoors to open, but this isn't always the case).
There are times where you can justify a tight time limit even if it doesn't play an obvious role, like the example above. To blow my own trumpet, for my "Faithful Friends" level, the correct solution leaves you with only 3 seconds, but you only run out of time if you are far too over-cautious with the digger or you let the rightmost lemming fall into an enclosed pit (at which point you can't solve the level anyway), and for "Sharing a Climber?", the correct solution leaves only 1 second on the clock, but it's only possible to fail if any lemming other than the first one out is made a Climber.
A good rule of thumb, I find... if you have quite a long level, make sure you finish it, on a bad day(!), with at least 45 seconds left on the clock; if it's tighter than this, consider adding an extra minute unless you are already up at 9 (in which case the level may be too long!). For short levels, it's up to you, but make sure you can justify the time limit.
For more general advice, start small. Many of my first levels filled only a single screen and had very little in terms of superfluous decoration, but at least two of them were good enough to include in the "Revenge of the Lemmings" pack at the expense of my larger, more ambitious levels, even though they are the best part of 12 years old. Smaller levels are much easier to debug and give you a lot more room to experiment with clever tricks.
Oh, and here are my personal dos, don'ts and general advice - some may be a bit complicated and won't come into play until you become more proficient, so don't worry too much if you don't understand them all:
- With the exception of crushers hidden in the ceiling, don't bury traps in the terrain unless it's obvious that it's there. This comes off as very unfair and a seasoned gamer will know you only placed them there to prevent a backroute, where something like a steel plate would have been more justified.
- Don't mark normal terrain as "steel", and don't place steel plates without a steel zone to complement it. This is very unfair and illogical.
- On most tilesets, the exit comes in two parts: a static door, and an animated flag or torches to go on top. Only the static door actually functions as the exit, but make sure you remember to put in both and line them up correctly, otherwise it won't look right.
- On some versions, objects with an index equal to or greater than 16 won't interact with the lemmings, so be aware of this if your level has a lot of water, say.
- If you have multiple trapdoors, the order at which Lemmings drop out of them depend on their index value, so make sure they are ordered as you desire.
- Avoid having more than four trapdoors if you can, because not all versions support more than four.
- Be careful when using three trapdoors, as some versions have two lemmings exit the second trapdoor for every one lemming from the others (i.e. ABCBABCB instead of ABCABC).
- Limit your level's dependence on precision bombing (using Bombers without Blockers), or if you must use it, make sure there's plenty of leeway, otherwise it just gets annoying.
- Don't make a level that relies on glitchy behaviour to complete; for example, a lemming climbing through a gap where logic should indicate he hits his head and falls.
- Don't give objects a negative Y-coordinate (partially off the top of the screen); this can crash the game sometimes.
- Avoid situations where a basher needs to bash near the top of the screen, or worse, partially off the top of the screen; this can crash the game sometimes.
- Make sure you understand which traps kill all lemmings on contact, and which ones only kill the first one that encroaches it before having to reset. Generally, traps that are always in motion (namely water, the shredder traps in the Marble tileset and the flame-throwers in the Fire tileset) will kill everyone indiscriminately, and those that are sitting dormant will only kill one at a time.
- Be careful when being precise with splat distances, as the critical height can vary between versions.
- Try not to do long builder levels - this gets boring very quickly!
- Try not to make it obvious where you've patched a backroute. Sometimes this can't be helped, but you always get extra marks for having a level that looks seamless and pretty without obvious patch-jobs.