I frequent a Christian forum that got a new member of its design team and it switched from the Simple Machines platform onto XenForo about 2 years ago. The XenForo format actually looks significantly nicer and has more features like being able to "Like" posts. Yet something about the "Simple Machines" format made it feel like it worked better for that forum, and I can't really place it. It was something about the simplicity and plainness that actually that made it feel easier to use. For example, there was a short, simple list of the newest thread posts at the bottom of the forum's homepage. Maybe the XenForo format has that too, but something about the XenForo format made it feel bulkier, like if they have paragraphs instead of just the titles in that list. But I am not being too negative about XenForo. The XenFOro format feels more fun and less formal.