-removal of technology trading
-removal of religion
-removal of espionage
-removal of civics (related to diplomacy, as far as we know social policies won't really affect this)
bjbrains partially said what i was going to reply to this in page 4 but i'll go ahead why i think diplomacy has a chance to be even more exciting and deep.
First lets take these two aspects:
in civ4 there are: religion, civics which give flat bonuses to like/dislike to various nations. You choose your religions/civics based on civs you want to like/dislike you. (and the bonuses you get from civics yourself)
in civ5 those two are replaced by :
policies which seem to give flat bonuses to like/dislike citystates which in turn most likely will modify your relationships with other civs and probably in much less predictably. You will choose your civics based on the citystates you want to like - but what happens if this citystate gets in conflict with the nation you like - decisions decisions.
In multiplayer citystates can actually change diplomacy between players - no player (except maybe a roleplayer) will care what civic or religion you have - but what if a citystate gives you some sort of fat reward to attack the player you planned to be friendly with for time being.
civ 4's technology trading will be replaced by more indepth strategic resource trading - now when you have 4 iron, giving one iron away means more then it did in civ4. Its not the same thing but might open up as much depth as tech-trade without the exploitable setbacks.
removal of espionage im 95% sure that espionage or some other strategic layer will be added in an expansion - for most people its no loss, and for people who will miss it - well you just might to wanna play something else until expansion gives your depth layer back
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After finally reading up all the details we have on it, the only two simple things about Civ5 for me seems the excellence of everything i have seen/read so far - and the unparallelled anticipation i have for it.