Is part of what makes acid-moss good the fact that you can ramp it out turn 3 with an elf? I find it so interesting that it has such a following, while other 4cc LD is basically unplayable.
Might 3 mana LD be too good if you are making the same play?
That is probably part of it. Because it's GG, I almost never see it outside some kind of ramp plan. On the play, if you elf into this on turn 3 and your opponent can't kill the elf, you often go into T4 with 6 mana with your opponent still stuck on 2 mana. I can't remember the last game I lost from that position.
What I think is amazing about this card is the fact that you are doing two very useful things simultaneously - ramp (and fix if you have shocks in your deck which you should) and setting back your opponent a turn. In some cases, it's better than plow under (which is an all-or-nothing version of this card IMO). Acid moss is still very good later in the game too because it usually has a nice target (like man lands or annoying shit like kor haven) and it can very often get you that 6th/7th land for your fattie (especially useful if all your elves have been killed). The card just rocks, plain and simple.
Cards like Stone Rain are strong enough to see play in my cube (very strong on T2/T3 obviously), but are often just dead late game (without a juicy land target). I think too many land destruction affects and LD.dec becomes viable, which I can tell you would not go over well with my group. So I don't run many of them.