Thanks all. The more I see of XF the more I like it - although there are a few things I'm not keen on, but hopefully I can find work arounds and make do without the features it doesn't have that I like (number of people viewing forums, thread icons, subforums shown on forumdisplay etc).
On a forum I converted awhile ago, the admin installed an add-on that added the list of users browsing a particular thread. So even if you can't find exactly the right add-on at first, it should not be very hard to add or create. It takes some digging to find add-ons, but you may want to peruse the list to see what you can find.
The only thing I could mention is that on a "big board" forum (hundreds online at a time), additional info will add at least one additional database query. My vB big board is at capacity right now, with our database's disk at or near 100% i/o capacity during peak hours, and it's getting slow.
As for subforums, I am pretty sure there are a couple of options for displaying them. I'd have to check, as I generally do not use them.
Overall, I can summarize my XF experience: end users love it (it is clean, easy to use and very engaging), and the admin side is not a trainwreck like vB. I can easily get around XF; I've run the vB big board forum for seven or more years now, and I still dislike (even dread) that horrible admin interface. Its main problem is that so many features have been added on, it grew into a rather uncontrollable mess.
Looks like it's time to get my read on then. Thanks for that Razasharp. I just skimmed through the message provided in your discussion and it isn't all that hard to understand.