Okay, ugh, now to talk about what is or isn't a glitch:
* nuke glitch is a glitch (nuking the level before all lemmings are released, the game only counts the percentage based on lemmings released so far, not total number of lemmings the level gives you)
* direct-drop is
not a glitch (a lemming landing on an exit trigger from a too-high fall it would normally splat and die from)
* using blockers to cancel out the effects of steel areas and object trigger areas is a glitch
* using the trick of separating the blocker's field from the blocker lemming itself is a glitch
* okay, here's one that's a pain to decide: terrain pixels being removed that falls under a steel area or the trigger area of a one-way-wall. The problem is that Lemmings, unlike Cheapo. fundamentally does not work that way. Terrain pixels removal is an all-or-nothing proposition, and the decision rests on things like whether the lemming was standing on steel, or has steel/OWW in front of him at a certain distance.
There's unfortunately a whole spectrum of examples, some of which most would agree glitchy, others not so clear cut. Since right now we say we don't care about any glitch solutions that uses 2 or less (or 7 or less) skills, we could take a hardline approach and say that in the 3+/7+ skill solution with no glitches, you're not allowed to remove any terrain pixels that falls under a steel area, or the trigger area of a one-way-wall when the lemming is facing the opposite way. Note that this is much stricter than usual, since something like having a miner turn when he mines into steel, that can easily break this "strict" form of the rule, since a few pixels covered by the steel area is removed at the miner stroke that hits steel.
Other approaches is to either say we don't care, or have some really detailed conditions like "at most one stroke of the miner/basher/digger can remove terrain pixels covered by steel/OWW-the-wrong-way". This still leaves the question of exploders.
* all forms of miner-positioning glitches are not allowed. Even this one is really hard to specify exactly, but we'll say that it's a glitch if there is ground for the miner to stand on, but he falls anyway because of the positioning error. There's also the issue that the miner can skip over one-pixel-wide gaps because of the way the game checks for falling for the miner.
* using blockers to push lemmings into or through walls is a glitch
* the sliding glitch (making a lemming jump up a 6-or-less-pixels-tall step, but using a builder to turn it into a taller step before the jump is complete, the lemming keeps jumping nonetheless) is a glitch
Ugh, have I covered everything yet? Probably not.
For example, what about the various cases where the game lets you assign skills while the lemming is standing on thin air? What about the lemming started off on ground, but while carrying out the skill (eg. bashing or building), the ground is removed in the middle of laying the build brick or removing terrain?
You can begin to see what a nightmare it is to fully define what is or isn't a glitch. I think the only way to handle this is to say that one tries to avoid the glitches, but leaves it for namida to decide whether something's a glitch or not, within reason. I know this smacks of subjectivity but I see little other choice.
Hopefully, in practice this might not be much of problem, given that many of the glitches you would probably need to go out of your way to enable it to happen in a level.