At the risk of derailing this thread further, my experience is the opposite (even though I did use
both 1e and 2e and later 3e and 3.5e interoperably). That is to say, I felt like I needed to change less to incorporate 1e stuff into 2E (or vice versa) and it was very easy than I did for 3/3.5e.
3.0 material works fine with 3.5e in the sense that it doesn't "throw errors". The only thing I can think of off-hand that would are differently-sized weapons*, and that's pretty easy to change on the fly. However, some things were
rebalanced for 3.5e – many spells had durations changed, some spells were moved to different schools to balance them better, monsters got more feats, some magic items had their costs and/or effects changed. You also had some rejiggering of classes, notably rangers being redesigned as per tradition, and druids getting spontaneous conversion of prepared spells to
summon nature's ally thereby making them amazing summoners. But there's nothing that prevents me from taking a 3.0 stat block and using it in a 3.5 campaign. It might be over- or underpowered, but it will work fine. There might be some issues using 3.5 things in 3.0 because 3.5 IIRC added some new concepts that don't have a 3.0 equivalent – particularly if you look at the horde of new classes 3.5 added.
* In 3.0, weapons had an objective size, and the difficulty of wielding one depended on the relation between weapon and wielder size. So a short sword was Small, a longsword Medium, and a greatsword Large, which mean that for a Medium-size wielder they would be light, one-handed, and two-handed respectively. You had rules for changing weapon sizes and what effect that would have on damage, so you could in theory make a Medium-sized short sword that would deal 1d8 of piercing damage if you for some reason needed an ogre dealing piercing damage. In 3.5e, weapons had both an innate handedness and a size, but the size referred to what size wielder it was intended for. So a human could use a light short sword, a one-handed longsword, or a two-handed greatsword, but all of those would be Medium weapons. A halfling could also use a light short sword, a one-handed longsword, or a two-handed greatsword, but those would be Small weapons and do less damage than the Medium equivalent. You could use a wrong-sized weapon, but it would modify the handedness by one step per size difference and also inflict a -2 penalty to attacks per size difference. So a human could wield a halfling's greatsword in one hand, but at -2. Basically, the idea was that a greatsword isn't just a longsword that's been scaled up, but it's built fundamentally differently and if you use something that's built like a greatsword but sized for a halfling, it's going to cause issues.
