Some questons/comments/concerns:
1) Why not keep {{}} for inclusion to be consistent and use <<>> for style?
2) I'd keep HRs to exactly ---- (4)
3) I like getting rid of he single bracket or external links, doule brackets are good enough for this.
4) I don't like keeping magic links (e.g. ISBN), just because they show up out of nowhere (i.e. magically). Why not [[ISBN:1234...]]?
5) I don't like superflous named arguments for links all that much as it's increasing the complexity (more than one way to do things, it will confuse people, more stuff to translate, rememer, etc)
6) I don't like the type for inclusion being separated by space, but that's not all that big a deal (I would like the : to be used for inclusion as well <image:blah.jpg>)
7) Don't like multi line inclusions, but if you have to do it, I would use this:
<<begin applet applet=conway_life.class size=400x300 pattern=none
meaning no comments, and only one set of opening and closing signs.
8) In the raw example, where does <<raw end, because math is getting interpreted?
9) I *really* don't like styles using, and especially using {{}} for them. I know many people, including myself, who are used to the current syntax will bug out at that. Why give one more choice, instead of using the the other markup? If people want to use markup, fine, let them use full HTML (for MediaWiki, I'd be OK with not enabling HTML for Wikimedia). I don't want to open up an artile for editing and have to figure out what {{u}}, {{i}}, etc do, especially when they're nested. One set of markup should be enough, for anything else use HTML.
10) Again, for the styles I don't like one set to begin the markup and one to end it. I know it makes the syntax easier probably, but having to hunt down to which style an end tag belongs would be a pain in long articles.
11) I really like the ////. ****, $$$$, ____, ^^^^, but I don't like terminating them by spaces, require begin and end for consistency.
12) Not sure how I feel about the character entities not being interpreted. I always hate seeing them as it makes editing more difficult, but I don't know if they are needed in some cases. I personally almost never use them. Some people may have trouble finding the character to copy and paste when the keyboard doesn't support it.
Thanks for taking a big step towards cleaning up the syntax!