I'm going to take issue with the comments about spacing. There is no question that the tighter spacing in the East sharpens ski skills. However, there are 2 major downsides:
1. Really tight spacing can can only be skied in good quality powder.
2. When there are only a few skiable lines, they will get tracked out more quickly.
I would guess that the elite of Eastern skiers, probably well represented here, would argue they get their powder fix consistently in the tightest glades because 99+% of skiers can't handle them and thus stay out.
But I am much in favor of expansive forests with enough spacing that you can ski anywhere. IMHO these are in relatively short supply even in the West. Typical of many western areas is a nicely spaced subalpine zone of only 500 vertical or so that closes out when you get lower.
The region which has long consistent fall lines in the trees is the Kootenay region along the U.S./Canada border, and the Selkirk and Monashee ranges north of it. Fernie is probably the best lift served example, and Island Lake Snowcat is even better. For you easterners who want more challenge I recommend Red Mt., where the length and steepness of glades are far more than you get in Vermont. For less expert-oriented trees I recommend Schweitzer and particular Big Mountain just across the border in Idaho/Montana.
At many heli operations you feel restricted if snow, visibility, etc. keep you below tree line. But on poor weather days at Selkirk-Tangiers and CMH Kootenay there were consistent tree runs of 2,000+ vertical all day long. I hope to report on the Monashees next February at Wiegele
. Judging from Fernie pictures these big trees in forests are spaced due a brushy alder undergrowth that gets well buried by January in normal snow years.
There are a few isolated areas with good trees like Steamboat, for which I have no obvious explanation. We have great tree spacing here in SoCal because that's the only way they can survive our hot and dry summers. Unfortunately there is only enough snow to take much advantage of it every 3-4 seasons.
I will be very interested to get Marc down here to Baldy when it is well covered like last season, and see if he agrees that its topography and powder potential is a close analogy to Mad River Glen.