UngeheuerLich
Legend
... and a lot of feats and spells and a lot of little things you only notice in direct comparison...Which was true of 3e and 3.5e too. The vast majority of the changes were in the category of what would now be called update, the actual mechanical changes were few and far between.
You could play monsters with a 3.5 DR with 3.0 characters under 3.0 DR rules and vice versa and you wouldn't have any technical problems. Sure, there might be balance reason due to the different philosophies (the 3.0 guys wondering why one would bother to assign such a low DR with such an obscure material that their +1 weapon beats anyway, while the 3.5 guys would curse whoever assigned such a high "all or nothing DR" with such an obscure material) behind, but no technical barriers.
You could play a 3.0 ranger beside a 3.5 ranger (although one would wonder why someone would want to be the 3.0 ranger ins this case), there was no game system barriers preventing it. Or take a 3.0 PrC unchanged for your 3.5e character, also no technical barrier just a balance issue due to the different philosophy (e.g 3.5e disliking the 17-20 thread range the 3.0 PrC just granted you). The only thing which you basically needed to change to make classes compatible would be a couple of skill names.