It seems "constricted" is the standard transition they now use from zone to zone, a narrow gorge, canyon or passageway. About the only ones they got right were the wide passage into Enedwaith & tunnels into Moria.
Well, since not a lot is going on here, may as well give this some scrutiny.
There aren't a lot of these "transitions" discussed in detail in the book, are there? I agree that many in the game are very small points of entry, for example Rivendell north to the Misty Mountains (which is one of my favs, that long climb up where you shift from the sultry river valley into the cold clime of the mountains, and subtly). I remember it took me forever just to find the path into Eregion, which was frustrating but also really cool at the same time, almost a quest all of its own.
I think the path from the Midgewater Marshes into the Lonelands is relatively broad and interesting, and though the terrain is mentioned in both the Hobbit and LOTR, is not discussed in a lot of real detail (I think the Hobbit passes over it in a paragraph or two IIRC).
I was frustrated as a low level character to have the (narrow) path north into Forochel barred by those bandits, back when the game was difficult, but it gave one something to look forward to also and I would not change that. I actually miss the days when there were places you couldn't go at will. They are now fewer and farther between, though my perception is coloured by my levelling up as well.
The choke points seem to make sense from a game stand point; I suppose they help to divide server load also - ? - RW can probably speak to that, I don't have a clue. They do seem artificial and give you that "rat in a maze" feeling of a FPS, but with a world so big, seem like less of an issue to me on this scale. And there are enough wide open transitions (go from Bree to the North Downs, for example, and you can get to Kingsfell two entirely different ways - via Trestlebridge, or go the long way with a right hook over the Downs, past the turtles and then a long swim) that it makes up for the narrow choke points.
But I can see your point that a broad transition as famous as the Gap of Rohan should probably be one of the wider ones - but I'll wait until we see the final release before passing judgement. I do think that in general, all these expansions will tend to be smaller and smaller, and nothing as expansive as Moria, which will be too bad.
