Floodplains turn 0/0/0 desert tiles into 2/0/0 (or 2/0/1 if the tile is adjacent to a river). This is a mechanic that makes an otherwise hostile patch of desert into a viable city location.
In a way, how this mechanic is portrayed in the rules balances well imo both intuitively as well as game play.
From an intuitive standpoint, flood plains form a relatively thin strip of super fertile land, while grassland represents much greater area of arable land.
Game play wise, without sickness to balance the extra food CIV4's flood plains were bestowed, the tiles are more inline with grasslands when taken out of context of the larger game.
Keep in mind that grasslands cannot normally contain wheat; the only farmable food resource in Civ5. Floodplains and plains are the only tiles which can receive wheat per the rules and minus world builder/ custom map script antics.
So freshwater based tiles with a farm, best final food. Keep in mind farms adjacent to fresh water get +1 food with civil engineering (+1 food with fertilizer for farms not receiving civil's bonus.)
Grasslands 2base +1 farm +1 farm/civil = 4 food
Plains 1base +1 farm +1 farm/civil =3 food
Flood Plains 2base + 1farm +1 farm/civil = 4 food
Plains+Wheat 1base + 1farm +1farm/civil +1wheat =4 food
Flood Plains +Wheat 2base +1 farm +1 farm/civil =5 food
What really broke flood plains in civ4 though was the synergy created when playing as an financial leader. That little combo turned FP's into 3 food +3 commerce right off the bat, circumventing the early game tradeoffs regarding the slow maturity of cottages.
Civ5 lets you farm on any normally passable land tile (the rules say "anywhere but ice" but eh, that makes no sense.) regardless of access to fresh water. This is balanced in that farms next to rivers lakes and oasis are +2 food by the medieval era, while the land locked farms need to wait til the late renaissance era to get that additional +1 food.
Overall, I really like the changes in farming, as no longer is an expanse of dry plains only marginally better than tundra or desert. Play civ 4 on an ice age map to really get a taste of the despair endless plains can invoke and you'll get an idea of what I'm speaking of.
Cheers!
-Liq