One change coming to Fourth Edition D&D is how monsters are being handled. In the Third Edition the default assumption was a group of four player characters versus one monster. Fourth Edition assumes a one to one ratio of monsters to player characters. Personally, I think this is a great idea.
Another important change is that monsters will have a greater range of levels where they can still be dangerous to adventurers. In Third Edition, if you were off the base challenge rating by just a little bit, the encounter quickly became either impossible or a cakewalk. Perhaps this sounds familiar: "Well, can he even hit an AC of 28? Just roll to see if anyone gets a 20 then. Nope? Ok, I make two attacks. Can they survive my minimum damage? Ok, just remove them from the board."
Even more interesting to me though is that they are encouraging Dungeon Masters to mix and match monsters. By giving monsters roles like "masterminds', 'lurkers', or 'brutes' they are encouraging DM's to treat them like an adventuring party. Few people put together an all wizard party with no "meat shields", so why would the DM put together an all Mind Flayer encounter?
I really believe these changes will open up the door for some truly massive and diverse battles. Remember the scene at the end of the movie LOTR: Fellowship of the Rings where the Fellowship fought off a huge group of Uruk-hai? Tough to do in third edition, easy to do in fourth. Heck, if you are really feeling saucy, improve on Tolkien by adding a little monster diversity into the battle.
Did I just say improve on Tolkien? Crap, here comes the hate mail. I guess I better quit while I am ahead.