More than saying BSD is not Linux, I'd say, it's not Ubuntu. That's not a crack at Ubuntu. Their aims are different. Their stated number one bug is that Windows is more popular, so they aim at changing that.
The biggest difference that you'll find, I think, is that rather than using GUI (graphical) methods of configuration, you'll be editing text files. OpenBSD is known for being very well documented, and much of that documentation is easy to understand, even for a newcomer. For instance, in installation, you'll have to manually partition your disk with what seems an intimidating amount of typing text. However, once you do it once or twice, it becomes quite logical. The instructions on that are VERY clearly written.
The book you mention is also very good, and Mr. Lucas tends to explain things well, even for newcomers. However, as has been said several times, don't neglect the OpenBSD faqs, they are also written very clearly. Some of programs aren't that simple, such as pf, but they are well explained. (Also, there are various guides to pf around that explain it in simpler terms.)
As for these forums, having been around Ubuntu forums from time to time, I'm not sure of what you mean by being overly moderated. That's a huge and busy forum, and so the moderators may have to keep a tighter rein than our excellent moderators do. However, from what I've seen of those forums, there is very little cause for complaint.
Most of the time, the complaints I've seen, from both Ubuntu and another very busy, but well-moderated forum, Fedora's, come from people who are aggravating everyone else.
One difference between these forums and Ubuntu's also stems from the different objectives. Since Ubuntu considers Windows dominance of the desktop to be their number one bug, their forums will show great tolerance for newcomers. We also welcome newcomers, but are a bit more demanding of them. For example, a question showing that you haven't even looked at easily available documentation may be ignored, or, at best, answered with a terse statement that you should check the OpenBSD faq and come back and ask if there's something you don't understand.
On the other hand, I've never seen anyone flamed for saying they read a man page and it's over their head. One difference you will find between some forms of Linux and the BSDs is that in general, the BSD (and this includes all of them) seem to take more pride in writing clear documentation, whereas many (though not all ) Linux man pages seems to be written by a coder with the attitude of, "I wrote the darn program, now I have to document it too? Oh, alright, let them use this."
In fairness, many distributions, including Ubuntu, provide their own, quite good documentation.
Like Corey_James, I want to my add my support to Bienc's statement. It should probably be a slogan. It will be a lot of a work and require a lot of studying to attain what you want with an OpenBSD desktop, but if the bug bites you, you'll enjoy it (sometimes).