This I like, SEO is incredibly important for me and well structured code is a massively underlooked aspect.
Thanks for all the other responses, I guess they were the answers I was expecting. It is difficult to have killer features in this day and age.
I suppose it would be better to talk specific comparisons, especially in regards issues I have with IPB.
While people have commented on it's lack of add ons, what thoughts do people have on the ease of writing add ons and integration with other scripts?
In terms of Forums Scripts I found IPB to be very bloated and mod creation difficult. A big turn off is the fact that I can't quickly and easily see much documentation for extending Xenforo. Any forum that could create the sort of documentation and example code libraries that the likes of Google do for their API's would get a massive thumbs up from me.
I'm a massive fan of the Code Igniter community and while the framework itself has some issues, I love the fact that rather than finding people who've shared fully made scripts, people tend to share frameworks. That is, tools for doing what you want rather than providing "what you want", which is never actually "what you really want".
Any forum script such as this which could build a community aimed at sharing between developers would probably become the best community out there. If you look at any others everyone's making add ons that can be installed in one click by novices, not sharing within a community of developers. Can you imagine ever taking an IPB addon, adjusting it a bit and re-releasing it? I can't, all hell would probably break lose.
But i've gone of on a tangent, what I mean to say is, what is the developer community like here and is there much in the way of code examples/libraries and if so where the ruddy hell is it beacuase I can't find it
On a final note another big thing for me is not just speed, but general server load. IPB seems to use 20-25mb of memory on every page load (with APC cache) While this is on a forum with nearly 200,000 members it can take its toll on the server with over a million visitors a month, to the point that I now have 32GB of RAM on my machine. But I think this was already covered when I last looked at Xenforo a year ago!