I like the diagonal walls. Not too convinced on the font though. I think it's too small for serif to work well. Mostly it looks alright (apart from the 'b' as mentioned, but the uppercase 'W' as well), but if you compare it to the old font, I think it's better.
Have you thought about how the diagonal walls will look with trees in the next version?
I'm thinking about using crenelated walls for my own tileset.
I looked at the 'W', but it's not really fixable without making the letter sans serif, which will look out of place, or by pushing the letter outwards by two pixels. This makes it no longer align with other capitals, and is therefore even more noticeable.
Personally, I think the font works quite well, even at a small size. The 'b' is fixed, and the 'W' isn't that bad. When I'm not working quite so much (in a day or two), I can post some sans versions if you like. The number of tilesets might get unwieldy; I would then be up to 8 of them.
As to the trees, it should be fine, although the more traditional tiles will work better. Larger trees will look much more natural, as the corners will be nicely sloped, but any 4x4 trees will be diamond shaped. The branches will be further from the trunk than they are in the version with corners, but even then, I don't think it'll be very noticeable, but I could end up biting my words.
When I have some time I could throw up a picture if you like. It will be a good representation, provided that the multi-tile tree screenshots Toady posted remain accurate. I think both types will fare better than the default tileset, which has trees with rather sharp looking corners. Reminds me of Minecraft. Still much better than single tile trees, so I'm not complaining.
I like crenelated walls. It won't work well for trees or rivers, which is why I haven't used them, but most people don't seem to mind that, so it'll likely work well. Looking forward to seeing it!