Its' hard to say what XF's most impressive about xF. There are just so many great things about it, so I will just mention only the first few that come to mind.
The first thing that comes to mind is it's SPEED. I never seen software run so fast. I just don't mean that only as a "forum software", I mean that as in any PHP software period. From what I seen about 1.1 fixing to come out, it's going to run even faster.
The next thing that comes to mind is the alert system. User's love it and it helps keep them engaged in the community. It's good to see something like this to built-in because it gets my user's away from FB long enough to check their alerts at the site.
The code used to power xF is up-to-date and with the most modern techniques used, but as backwards compatible as it can be. This is something that most people don't take into consideration. Mike, Kier, and Ashley did a great job planning this out and executing it. A bunch of damn geniuses trying to take over the world one forum at a time.
I think the default theme needs some tweaking. I used to think it was beautiful, but now find it blurry and full of empty space. It reminds me of the SMF curve theme. Everyone thought it was awesome when they first seen it, but later realized it was bland. There isn't enough contrast between the colors. The thing I like alot about the style though is the breadcrumb. Wish it was bigger like Flexile. Another positive thing I can say is that I'm also glad the style isn't full of icons everywhere like most software.
The only thing I don't like is the menu. Actually, I hate it with a passion and consider it horrific. I don't like the idea of a mouseover open dropdown, but a click to close it. I often find myself opening it on accident, or clicking on other links trying to close it. I'm sure I'll get used to it eventually, and not have any troubles. Although, I wonder how many of my user's have the same problem?
1.| Major: option to display a topic as an "article", "page", "file", "thread", or "blog". Really if you think about it, the only different between the four is HOW the information is displayed. Selecting between the five options merely changes which template can be used to display that information.
Group permissions determine what type can be used, along with what code such as BBC, HTML, or PHP. An example of default restrictions would be Page & PHP is allowed by the admin only.
If your wondering what "file" type is, then it's nothing more than taking a post with an attachment and display to look like a item download page. All the information is there, it's just a manner of how to display it.
It would also make it easier to make an add-on for it. Someone might want a custom "reviews" add-on, that mainly just displays the information within the post to look like a review page.
All content of any type can all be found within the forum. No need for separate sections. Although if someone wanted it a separate section, then they could use a mod or look at suggestion #3.
2.| Major: option to make any age, gender, or custom profile field into a Group, so that it can be given permissions. This would allow sites to restrict content by age, gender, or custom profile field. This is something I have not seen any software do, but is always asked for as a mod/add-on.
3.| Minor: when linking to a forum category such as "http://xenforo.com/community/#public-forums.17", it should show only that category and the boards & topics within, not the whole forum index. This would give the admin the power to make it appear as if there are multiple forums on one site, but at the same time all forums can be found on a single page if need be. Also give the ability to apply a style to a category, and all boards within can inherit.
4.| Minor: I think that "Discussions" & "Messages" should be changed to "Topics" & "Comments". I also wish that something else was used instead of "Style", as that keeps reminding me of CSS. Everyone knows the common alternatives: theme, skin, and template, but an uncommon name I would like to suggest is "Templet".
Just my 1/2 buck.