It's easily recognisable even when small which is important for a logo and a favicon. I like the colours. And clearly it shows an amazing degree of artistic talent, the sort that could only be acquired in art school :)
Allow me nonetheless second some other commenters who observed
- the #\a concept is not obvious
- the keystone would normally be part of the arch, not under it
- the blue and black are difficult to distinguish in the small version
- while you explain the #\a and the keystone, the red/blue business is mysterious, and perhaps a tad decadent - art for art's sake and all that.
At a glance, it looks like the white bit has risen up out of the red bit (if you'll forgive the technical language here), and the blue bit is just background.
A slight improvement might be to make the horizontal edges of the #\a (top and bottom) a little thicker, to enhance the a-ity (although I can see that that might diminish the arch-ness). Another might be to use a brighter blue.
Are you planning to stick with this or are you inviting suggestions (not that the absence of an invitation has stopped me or anyone else) ?
My wife (a fine artist) liked the aesthetics and said it reminded her of a Mondrian painting, but she didn't get the lowercase 'a' or keystone :)
After your explanation, I do see the lowercase 'a'. But, it does bother me a little that the 'keystone' is rectangular vs. a wedge, as it seems like it might slip out of the arch.
I also see orthogonality and (as another poster said) a floppy disk.
Probably the most prominent image is a square which unfortunately doesn't seem very arc or arch like to me.
There seems to be a lot of potential with arcs, arches, 'A', lambda character, etc. Also, having a logo change is not a huge problem, so not getting paralyzed with an attempt to get the perfect logo initially is important IMO.
Recognizable, simple, unique, and scalable. Sure, it works as a logo.
I didn't see the arch and letter motif until it was explained. I think this was the case for most people. It's much less than subtle.
In my mind, an arc/arch is all about the curve; it's the fundamental and universal shape of timeless strength. (I'm guessing that's where the name originated.) Because of that, I'm having a really hard time connecting a square, blocky image to that idea. Orthogonality != Arch.
Icons need contrast and primary colors are a fair choice. But I'd tweak them a bit. At first glance, I was reminded of the default MS Paint palette or an maybe 8-bit Sol Lewitt. I think I'll help with the blue/black scaling issue too.
Arc isn't finished, and I'd guess neither is its logo. LISP (and its dialects) historically have had very atypical logos, so it's refreshing to see something new and thought-out.