I can see how it could be confusing to find those resources if you're starting from this forum, but it's worth noting that the site's home page [http://arclanguage.org] links to them.
I'm not the site admin, to be sure. I guess that would be pg or rtm?
One usage for that dot I like is with Anarki's 'src and 'help. Being able to do src.foo for (src foo) and help.bar for (help bar) makes it that much more convenient to resolve doubts.
It's needed for multiple variable assignments. As you've noted, for a single variable assignment like (= x 12), (assign x 12) is as good as ((fn () (assign x 12))). But for a multiple variable assignment like (= x 12 y 34), whose expansion is ((fn () (assign x 12) (assign y 34))), taking out the thunk causes an error.
Posting here as you've just done is a good start. :) You might also ask on Hacker News if anyone is interested. Can you say more about the project and what sort of "vertical" you're considering?
The original "Arc Challenge" thread [1] has a lot of information about Arc vs. other languages, including lively debate about the merits of their respective differences (and about the merits of the competition itself).
GaryLensman has a provocative comment in the subthread http://arclanguage.org/item?id=858, in which he argues that Arc is essentially a custom DSL exercise routine to PLT Scheme programmers.
There was a post on Hacker News yesterday comparing various Lisps, but Arc wasn't included. [1] I actually started a column for Arc in the comments, but it's incomplete and probably has some inaccuracies. [2]
Partially because it was what I used when I was first introduced to Scheme. It has the basic Lisp-editing features (paren-matching, indentation); it colors text (e.g. comments are orange, literals are green, normal code is blue); it has a nice GUI for customizing much of the above (for indenting, you can give it specific words or regular expressions to determine whether to indent something 'define-style, 'lambda-style, or 'begin-style; you can also pick your own colors instead of the above, though I don't do that). I also like that cmd-I reindents everything.
Drawbacks: Indentation can't be customized as much as I'd like[1], at least as far as I know. Tendency to freeze up for several seconds (maybe because I tend to leave several files open for days at a time, but I'm not changing that).
I've experimented with Emacs too, and they are pretty comparable. I use DrScheme these days mainly out of habit, but one obstacle in Emacs is that I don't know how to make it do Arc-specific indentation, whereas I can do that pretty well in DrScheme.
[1] For example:
(with (this (blah)
that (thing)) ;this line is too far right
(do-stuff) ;this line isn't, because I told it
(do-more)) ; to indent "with" lambda-style
