None. Why bother with a mix of 3.0 and 3.5? I haven't seen a rule that got worse in the revision (IMO, of course). Some changes seem unnecessary, but I figure it is easier to just use everything in 3.5 instead of keeping a long list of exceptions. I mean, I started playing 3.0 with a long list of house rules and dropped most of them because it wasn't worth it - why should it turn out different for 3.5?
However, in the group I play in (not DM for) we discussed converting in mid-campaign and two people were completely against it. Let's see how that works out.
However, in the group I play in (not DM for) we discussed converting in mid-campaign and two people were completely against it. Let's see how that works out.