I'm about 90% of the way through converting The Gates of Firestorm Peak (2e) to 4e.
The only problem that has really slowed me down is keeping the combats interesting. For example, on the way to the Gates, the PCs are supposed to encounter a vicious cave bear. Looking at the bear's statblock, it looks like he could possibly hit the PCs for a bit of damage, but the fight should be over pretty quickly without any threat of dying. The fight is there more for setting up the tone of the module rather than to be exciting. This gives me a lot of trouble since the fights in 1e/2e were so much faster than 4e. On the one hand, I want the encounter to be there for tone. But, on the other, if I put it in, chances are good that it will take too long to resolve with the new system and just slow the game down.
The older editions had so many more rooms and combats, where as 4e emphasizes fewer, but more dramatic combats. This is the hardest thing to deal with. Everything else so far has been managable.