Count me in with the group that is concerned about CIV 5 being "dumbed down" and less realistic than it's predecessors. While it may turn out to be a great game and have outstanding gameplay, the loss of realism (as being reported) it requires will be a major negative for me. Changes being discussed such as one unit per tile (on a strategic scale) and the loss of espionage and religion (both of which had major issues in CIV 4, but certainly can't be ignored in history) seem to me to be major steps backwards for the series.
One unit per tile has been discussed to death already so I won't say more about that, but here are the fundamental things I think could have been fixed about the other two that might have saved them:
Spying - a simple answer to all the spies floating around in CIV 4 would have been to assign a real negative effect to diplomatic relations from spies that are caught. A significant adjustment to the spied upon leaders attitude towards the spying nation would have been sufficient to drastically cut down on the amount of spying going on, and would have improved game realism. The effect should have been temporary instead of permanent (actually, all of the diplomacy effects should have been temporary - fixing the diplomacy system is one of the major positive hopes I have for CIV 5).
Religions - These were a great addition to CIV 4, but were hampered by a fundamental decision by the designers to have different religions automatically hate and distrust each other immediately upon creation, and also to have all religions distrust/hate each other equally (or close to equally - if there is a difference, I've never noticed it!). To really bring the point home - under the current system projected into the real world, Israel (for purposes of my point a "Jewish Civilization") would have the same diplomatic modifiers towards Christian civilizations as it does towards Muslim civilizations
While religous persecution and strife certainly has historical context, and there are some deep hatreds between certain members of different faiths, these things are based upon minimal initial distrust, magnified by years of history and sometimes mitigated by time and other events. For example, the Crusades and fights over the Holy Land have contributed to many Muslims distrust of Christians, while events such as 9/11 and terrorism have inflamed many in the West's distrust of Muslims. As an example going the other way, Jewish/Christian relations were sometimes very poor in history, but the two religions are generally friendly right now. The game needed to have some sort of historical context on the interactions each of the different religions, and take that into account when deciding the modifiers on diplomacy.
This improvement in realism would have also addressed everybody's prime gameplay concern about religion - it causes the game to be too predictable and breaks CIVs into blocks. The blocks would only form if there was a reason for them to, and would be variable every game based on the actions of the players or AI.
In my mind, both of these ideas are great examples of what I would see as a positive change for Civilization - improvement in gameplay in these cases would come as result of improving realism, not taking it away.