Conversion work is much harder in
4e.
Magic items tend to work completely different.
Levels don't align anywhere like they used to.
The Monster Manual is a gimp book and in dire need of support/reinforcement (not hard to guestimate the goods in the meanwhile but at the cost, with the reprinted art and reduced page count, I don't feel I should have to.)
I'm not saying any of these things are bad in terms of
4e being it's own game.
But when trying to do conversion work... it ain't as easy as it could be.