This is an update from my last response in another thread.
Some of the main issues are
-duplicate ID's for characters | items | clans | etc
-duplicate names for characters [with the name change system a server wide forced rename could be taken into effect] this one is now almost solved.
Between you and me, the biggest issue is line 1. It requires the most amount of work to develop a system, and verify that NOTHING at all is going to go wrong. Essentially all characters items clans etc would have their ID's re-numbered. And if it's NOT done properly, well you could end up logging into someone elses characters instead of your own, and not only that, but items could all be mixed up between characters that are now mixed up between accounts and clans would have rosters full of people that were never it in.
It would be a mess if it went wrong.
However, a merge has always been a +1 for me. Makes no sense to split the community. The only logical reason was server capabilities couldn't support the traffic there used to be.