Here is a running, compiled list of changes to the D&D 3.0 rules for the revised edition (aka D&D 3.5). Sources include: Shadowstar's scoops, Dragon, Dungeon, WOTC web site, WOTC Chat, Ed Stark interview, Andy Collins boards, Andy Collins, Ed Stark, and David Noonan via WOTC boards, and 3.5...
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Anyone who thinks the 5.5e change is right now anywhere close to the 3.5 list…feel free to take a gander at the change log for 3.5 i noted.
Now 5.5 MIGHT be as big once we get all the changes including the mm and dmg…but from just the playtests so far..:no it’s not close.
And beyond the amount of changes, 3.5 changed various fundamental mechanics, all the combat options, action types, OAs, jump mechanics, shields, damage reduction was overhauled, many skills, etc etc.