I think the biggest headache in the porting is anyone who uses {$varname syntax like Cardoe says. (FastTemplates uses this very syntax, so you either get the latest update or patch it yourself). very few people really get any incompatibilities in the upgrade.
The reality is a lot of admins aren't willing to risk it though. CI Host is filled with those types of admins. Not enough demand, so why should they do the work and risk having irate customers who's scripts may malfunction?
