1) I noticed you kept the "space for preformatted text" syntax. I think this is one of the single biggest causes of confusion, as it's quite easy to type:
One line ..
Another line ..
That is, to accidentally insert a space between two paragraphs. The output will look weird, you look at your source text, but you don't see what the cause is, as spaces are invisible. Unless you know about this particular bit of syntax and understand its purpose, you will likely leave the text as it is and hope someone else fixes it. I've seen quite a few decent articles where people made this mistake, but left it in -- though it's quite visually obvious in the output, it's non-trivial to fix. My guess is that if we did a survey, we would find that the large majority of users don't know that this syntax exists.
In my opinion, the leading space is not a big enough time saver to justify keeping it (particularly since it scales very badly-- preformatting two lines like that is easy, preformatting 100 is a PITA without search & replace).
2) Why not think about some alternative link patterns? I've personally always thought that double underscores might be cool. Compare:
During the reign of [[Charles I of Spain|Charles I]], who inherited the throne from his father Philip, '''Habsburg Spain''' controlled territory ranging from [[Argentina]] to the [[Netherlands]], and was among [[Europe]]'s greatest powers. For this reason, this period of Spanish history has also been referred to as the "Age of Expansion." Although usually associated with its role in the history of [[Central Europe]], the [[Habsburg]] family extended its realm into [[Spain]] from [[1516]] to [[1700]]. Under Habsburg rule, Spain reached the zenith of its influence and power, but also began its slow decline.
vs.
During the reign of __Charles I of Spain|Charles I__, who inherited the throne from his father Philip, '''Habsburg Spain''' controlled territory ranging from __Argentina__ to the __Netherlands__, and was among __Europe__'s greatest powers. For this reason, this period of Spanish history has also been referred to as the "Age of Expansion." Although usually associated with its role in the history of __Central Europe__, the __Habsburg__ family extended its realm into __Spain__ from __1516__ to __1700__. Under Habsburg rule, Spain reached the zenith of its influence and power, but also began its slow decline.
The benefit of __ is that it makes the text look underlined, which is an intuitive visual hint to hyperlinks--not to mention that having the opening and closing characters be identical makes it quite a lot faster to create links. (On German keyboards, underscores are also much easier to type than square brackets, but that's probably not the case everywhere.)
2a) You suggest supporting [[target=foo|text=bar]] as a more descriptive way of linking. However, I think [[foo<=bar]] or [[foo=>bar]] would be a more visually intuitive way to indicate the text/target distinction that does not require the use of keywords (it could, however, exist alongside that syntax, particularly with your title attribute and perhaps other link attributes that we might want to support, such as semantic information).
3) The template syntax needs to be shorter, since it's ubiquituous. People are not going to like having to type <<include NPOV>> where they previously could type {{NPOV}}. If you're trying to make templates easier to understand -- the biggest usability issue is not that people don't understand that templates are transclusions, it's that the content of "NPOV" is in the Template: namespace, but other than in the list of templates at the bottom of the editing screen, there's no reference to that namespace. I think that this is best addressed on the UI level, though.
My suggested shorthand for inclusions would be <<"stub">>, <<"NPOV">>, etc.
4) We generally have to be careful with hiding namespaces from users. <<image file:xyz.jpg>> may be better than <<image file=xyz.jpg>> or <<image xyz.jpg>> because it preserves the visibility of the namespace.
5) The style syntax, I think, is the biggest problem area of your proposal. I would actually prefer it to be taken out completely at this point and developed separately, as it is also a significant new feature with major implications. The main syntax-related problems I have with it:
a) This {{i may be {{b}}nested, as long}} reads horrible. Having styles separated from words with spaces makes them look like words. They are markup syntax, hence must be clearly visually distinct from the content they describe.
I think the problem is exacerbated by shortcuts like {{i}}this, where the scope of the tag is not intuitively clear, which may make it look like it's not a tag at all. I don't really see this shortcut as particularly useful either, as all frequently used styles should have symbolix shortcuts anyway..
How could it be made simpler? An XML-like syntax might be the most appropriate here: <red box>Warning!</red box>.
b) Using {{..}} creates *huge* transitional issues. Like it or not, templates are deeply rooted into Wikipedia, and changing the function of curly braces to something completely different will confuse the hell out of many users who use that syntax every day.
c) In any case, curly braces are ugly and programmer-like. I'd much prefer to get rid of them completely in our syntax.
There are other objections I have to using stylesheets, but as I said, I'd prefer to discuss this topic separately from the syntax.
All in all, however, this is an excellent proposal with a lot of potential. Transitioning to a new syntax will be painful, but the wiki-users of tomorrow would greatly benefit from the added consistency, not to mention that it will allow us to move towards standardization across wiki engines as well. Before this is finalized, hopefully we can also get some input from non-MediaWiki developers to find out what other features we may want the syntax to support.
Best,
Erik