Have you even played the game? Not only is there diplomacy in civ V, but (while there are some rough edges) it's *better* than diplomacy in civ IV.
In Civ IV, diplomacy was about using game mechanics to manipulate the other civs to behave in ways that were counter to their long-term interests. To make them act in a way that no human would act. To make them lose the game. In other words, to make them act irrationally by being your perma-friend, even though they could never win that way.
In Civ V, you have diplomatic relations with city-states and with other civs. Diplo with city states is more like civ iv diplo: it's extremely transparent and you can build up enough goodwill with CSs to make them your perma-friend. CSs, of course, aren't necessarily in competition with you. Diplo with civs, on the other hand, is more like diplo with a human; they won't forget that they are trying to win. On the other hand, there are lots of benefits to diplomacy - research pacts, trade agreements, etc. Civs will enter into these agreements if they seem to make sense to them - and cancel them when they don't. This is rational behavior. This is how civs should behave.
In my two games, I can't say I've noticed the other civs piling on me, though.