What are the fundamental design flaws of XL?

I haven't looked into it in detail but the design appears from a glance to be really good.

If you ignore the cost, which is really hard to do in this case. But that is also somewhat justified by them targeting businesses. It just means they are lagging (which is evident by how delayed mk4 was, if it had been on time mk5 would be imminent by now).

I do feel that the XL is a good stepping stone for their consumer line though, and up until the mk4 launch that is what I thought mk4 would be all about.