Having played and solved a considerable amount of level packs in the meantime, I give my 50 cents on this topic.
My turn-offs:
1. Boring and uncreative titles.
I dont' know how many "Bubble Bath" levels or something like that exist....For my taste, titles often stick too close to the level itself, sometimes even spoiling the solution. I like good puns. And I like references to the real existing world outside. Be it literature, architecture, music, sports, natural science or whatever. "The pit and the pendulum" by Geoff comes to my mind in this moment.
2. Vertical scrolling levels
I've got two problems with these. First, these levels often require a lot of jumping back and forth. Secondly, and more importantly, these levels often come along with a lot of Bridge Building. Going from the bottom to the top is self-explanatory since it's the only way to gain height, but it's also true the other way round when bridges have to be built to save Lemmings from being splat. There ARE great levels of this kind like "Gravity Coaster" by Flopsy, but most of them are not.
3. Levels with more than 100 Lemmings
What's the point in that? I just can't see it. For me, even 60 Lemmings would be enough for any puzzle. I've never seen a +100 Lemming-level that wouldn't be as good with a siginificant number of Lemmings less.
4. Levels with more than 4 hatches
That's especially true when this goes along with huge levels and therefore a wild jumping back and forth again. It's even worse when all (or most of the Lemmings) are in immediate danger. I find these levels not fun to play.
5. Lemmings with terrain-removing skills that have to cancel out or pass through each other
This falls into the "easy to spot, but hard to execute" section. It should be the other way round.
6. Bad music
Ok, that's highly subjective. I just can say that I can't listen to the original tunes anymore, so I'm happy with every pack that uses custom music. I admit I have a soft spot for Arty's packs, especially Sub Lems, for he used different tunes for every single level. Considerung tunes, I'm not a big fan of loops too - that's too predictable for my taste.
7. Confusing backgrounds
Not a direct turn-off, but I prefer a simple, traditional black or dark-blue level-background over all these landscape-like alternatives. It looks clearer and it makes it easier to tell terrain from background-objects.