Perhaps we could set a date for another meeting with many players?
Tomorrow is still weekend, so I'd suggest that day; I'm personally pretty flexible in regards to the time, so I suggest someone else could set a time.
I definately have to agree on the blockers, the issue with them is that they block the way instantly, and with many blockers available, things get easily pretty messy when trying to remove them.
There's also a tendency that most action seems to take place near the exits, and blockers serve as an easy last resort there.
The only good option I see is cutting down the number of blockers. While there also good instances for using blockers (e.g. separating one's own lemmings from the other ones), the ones I can think of right now can also be achieved otherwise, even though not getting the effect instantly might be a slight disadvantage.
I also agree on the diggers, though I found their power especially prevalent in the previous games we had with more than 2 players. It seems that with the L++ physics it's very difficult to release diggers using diggers of ones own. It's impossible to overtake a digger, as miners are slower vertically as well, and using builders to close the gap will for one rarely keep all lemmings up, and can also easily be destroyed again. So a digger is pretty much in control of the crowd around him. It seems again that cutting down their number is the only decent option for that. Or introduction of the Bomber skill from Lemmings 2 (though you'd still get down with enough of them).
Another advantage of the digger over the miner is that if there's terrain over the digger's pit, you cannot easily close the gap using builders (though with the platformer this can be prevented).
Whether climbers are useful greatly depends on the level though I think. Especially if it's difficult to access the exit by some other means, like in ONML 1, ONML 5 or Orig 3. In that case, the strategy would be to merely save the climbers, and ensure that the opponent won't get a different path to the exit. Of course it has to be ensured that the climbers' path won't be destroyed by the oppoent's lemmings too early, an ensuring this is an interesting task in itself. Also climbers can get out of deadly digger pits.
I find steel under the entrance/exit is not that vital, as building into the exit and hitting the trigger area is not as much of a hassle as it is in the DOS physics. It's comparable to fixing a bridge the opponent destroyed, from the builder requirement. Still it's not that nice having too much action near the exits.
One thing I noticed in the original levels, levels with paths where builders are potentially important (e.g. deadly bottom of level, or simply having to get upwards), some levels are pretty unbalanced between terrain removal skills and terrain creating skills, like having 20 of each which means 60 terrain removal skills and only 20 builders.
Yeah, the experimental levels with very limited skills were less interesting than I hoped them to be. I guess limiting/removing certain types of skills is more the way to go. Like ClamSpammer's miner- and digger-less level, which I liked very much with its multiple possible paths and the limitation of horizontal terrain removal. The blockers I used to delay my defeat could be reduced though.
Can't agree on 'the more players the better' though, I find 2P and more players have somewhat different type of gameplay, both of which I like. More than 2P sure has more action element, however maintaining some more control in 2P allows for more time to think about strategies.
I like however that more than 2P still allows for a balanced gameplay due to that aim of the game: In games where the only aim is to survive against the others, having more than 2 players frequently leads to inbalance, while in the lemmings games we had I haven't seen anyone team up against a certain player; only caring about one's own Lemmings and preventing any lemmings to enter a different exit.
L1/ONML mechanics is overrated. Just ask CustLemm level makers the bane of steel destruction glitches. 
Or the mystery of exits not working, levels crashing due to steel areas sticking outside the level boundaries, etc., before the days of Lemmix's level validator. And don't forget that there's Cheapo, which has been pretty popular for a while on the old old forums.
I guess some of us have grown fond of these peculiarities
, but yeah, they aren't useful for that kind of gameplay. I especially like the way steel is handled in L++.
