While Nightfall is a great game, it's still lacking in many areas that the original excelled at.
Something that adds to the gameplay is the new Hero system. They make forming groups much easier than before, especially for those hard missions in prophesies that you always had trouble with. The monks are essential for making it though Nightfall due to the constant lack of real monks. Getting your hero's skills set and equipped can be a huge hassle though. You must unlock the elites for them to use and skill organization can be somewhat time consuming. For veterans this will be easy, but probably not for newbies. As for equipping them, the sky's the limit. It is also simple to lvl them up, since they gain xp while you do too.
There really hasn't been any big changes in gameplay from the original. The PvP is still top notch in my book. Nightfall hasn't changed anything accept for the skills and races for PvP, which is for the best. Graphics are still the same as the original. The artificial intelligence has improved some, although it has become hard to break aggro lately, which is should be fixed in a near-future patch.
All-in-all, this is a great game that will keep you occupied for a month or two. To really get into GW, I suggest buying Prophesies, but for a casual gamer Nightfall will suffice. In game name: Mexican Beef