Upgraded from 3.8. to 4.1.4 and loosing traffic

I would appreciate some critique on this one:
Boggnasker - unge om bøger - Nyheder

Due to the massive changes in vb4, this is very common after upgrade. It should recover in a few weeks.

Originally Posted by Brian Cummiskey

Due to the massive changes in vb4, this is very common after upgrade. It should recover in a few weeks.

According to your experience what are the main items to look at closely to avoid loosing traffic when performing this upgrade?
This is the upgrade that I'm about to do but I wasn't thinking that it would make such a change in traffic because: links are the same, H tags are the same, forum link structure is the same overall.

I'd appreciate any info so that I can avoid issues where possible...

Just want to add that I regained my traffic again during time.

There is little you can do to avoid losing traffic. Any massive change will almost always end up with your site going through a re-indexing period. If you don't want to go through it, stay on vb3.