I would definitely argue against Climber and Slider being mutually exclusive, simply because of the interesting paths both can create in conjunction: A lemming can climb up a wall on the right, turn into a Shimmier to go to the left, slide down a wall on the left, and then at the bottom of that slide go back around the corner by transitioning back into a Shimmier. Combine this with the 2-way Jumper-Climber interactions and you can easily create paths that "skilled" lemmings can follow but the crowd can't.
And that is basically the point of a lot of levels. While I still get the feeling that "pioneer" is somewhat of a negative term here (because it has the connotation of the entire complexity of a level being reduced down to the actions of a single lemming while everyone else is contained safely), even a lot of the more challenging levels still require 2-4 "worker" lemmings, often in conjunction. And the fact that these workers can often take completely different paths than the crowd is what makes these levels significantly more interesting; because another thing that tends to make "pioneer" levels boring is if they're just carving out the exact same path the crowd will take later on.
One-way arrows, Swimmers, Gliders, Floaters, Shimmiers, all of these are common ways to allow worker lemmings to approach an obstacle from the opposite side, in order to create a path that the crowd couldn't create by themselves, even with enough distance between the lemmings - because the terrain simply isn't shaped in a regular lemming's favour.
Climbers and Sliders would provide many further such interesting interactions with each other, and those would only get extended even further by the combination with Jumpers and Shimmiers.
As far as L2 goes, if I remember correctly, the transition to sliding only happens at the point the lemming walked past the edge. Basically instead of transitioning from walking to falling, it would transition to sliding. Jumping a slider against the wall will not allow it to start sliding.
Ah, okay, thanks a lot for the information!
In other words: It's the "wall-jumping" part that's creating the problem, or, more accurately: The ambition to have Sliders interact with Jumpers as well, not just Climbers. For Climbers, it's pretty much a given at this point that they are going to interact with Jumpers both ways.
Personally, I quite like the idea of having to e.g. jump across a gap in order to reach a wall a lemming can hold on to that allows him to slide down an otherwise lethal drop safely.
That said, if we had to "ban" one interaction and I had to choose between Jumper-Slider and Slider-Climber (the latter by making both of these permanent skills mutually exclusive), I would probably rather stick to the L2 behaviour and sacrifice the Jumper-Slider interaction:
a) For consistency with L2 (for players switching from Kieran et al.'s custom L2 content to NeoLemmix, this can indeed be relevant!), but more importantly
b) because the Slider-Shimmier interaction is more important and revolutionary to me. Since the other main skill that interacts with Shimmiers is the Climber, I don't want to be forced to choose whether I can have only Climber-Shimmier- or Slider-Shimmier interactions with a given lemming.
But I think the case of the Slider being a temporary "movement" skill, like the Jumper and Shimmier as well (and the Walker, technically, counts too), can also be made, and actually only becomes stronger by going through these examples.Much like the Jumper assigned to a Climber doesn't stop the lemming from being a Climber, but merely tells him "stop doing what your new default state tells you to do, and do this now instead!",
a Slider would tell him "stop climbing for now and slide down instead, but once you reach the next wall, you'll climb up as usual / once you reach the next straight drop, you fall looking forward, like a regular Faller".
In other words, the Slider would become the logical extension to the Jumper, in that both of these skills would be the only non-lethal skills you could assign to a climbing Climber. As it currently stands, the Jumper is the only non-lethal skill that can be assigned to a Climber. But with that skill already being a precedent, there is nothing stopping other "movement skills" from behaving the same way.
If the Slider became such a non-permanent "movement skill", just like with Shimmiers, I think it should still be possible to have pre-placed Sliders in a level, as well.