Dormant registrations that; register, don't do anything for months, and/or no activity.

The bane of every Admin no doubt, in days of old, I'd email the accounts with pretty much like what you guys do at XenFans. A nice little "Where have you been" and "This is what has changed or been improved upon since you last visited".
Culling dormant accounts:
Yeah, we've all been tempted in times of doubt, and despondency; I can sympathise and see both sides of the argument but however try this epiphany...
1. Why make things so much harder for them next time around? Out of sight out of mind? You got it. They may have only just made it past the post with their initial registration.
2. Research, if you have the time to find out precisely "why" they're not active. They could subliminally trying to tell you something that otherwise you may not realise with your Admin's hat on. Been there done that, been amazed. Seen results. I kid ye not.
3. If you're blessed with a niche or local club and can physically see and/or talk to members. Get inside knowledge on their interests in AND outside of your core niche. Time for some off-topics geared towards their interests? You may just pull them back into activity on that and the core subject of your forum.