I think what "difference" entails needs to be defined before this question is answered. Obviosuly, there are more difference between DP and HGSS than there are between DP and Pt, but I feel like changes in "game mechanics" needs to be better defined. Like sumwun said, item locations changed, but I personally wouldn't consider that a game mechanic. Are locations of trainers a game mechanic? What if the trainers are in the same location but they variations in Pokemon (or their levels, moves, etc.)? Froggy mentioned move tutors, but I also wouldn't consider that a game mechanic, just something that's available in one game that isn't in another. That being said, there are other features that are available in one game and not another that I WOULD consider game mechanics, such as the Poketch, Pokegear, and Vs. Seeker. Would something like the differences in the Dowsing Machine count, as they have the same use, but the way you use them in DPPt and HGSS are completely different?
