Civilization – Why is the city-state influence I gain from gifts of gold decreasing in Civilization 5

This page has a number of interesting tidbits that aren't easily discoverable, including the one you mentioned above about Greece. Nothing else about civ leader specific abilities though.

You can skip the intro movie by hitting Escape; however, the game loads data in the background while the movie is playing, and therefore the movie won't actually stop until background loading is complete.

To access predefined maps, such as the Mesopotamia map that was part of some special editions, choose Mods and then Single Player. You can now select among all installed maps by clicking on the map button.

To show the movement range of a selected unit and the path it would take when ordered to move, either press "M" or hold down the right mouse button while moving the mouse cursor over the map.

The maximum radius within which a city can allocate workers is three hexagons in any direction.

Marble speeds production of all wonders by 25% in the city that works the quarry.

Greece has the special ability that moving units through the territory of city states never degrades influence with them.

Railroads not only give the usual gold & movement bonuses for roads, but also +50% production for cities connected to the capital. Harbors act as railroad connections as soon as the technology is available. The exact movement bonuses for roads & railroads are unknown.

Units count towards a supply limit defined by your cities and population, shown on the military overview. The exact effect of reaching that limit is unknown.

Units have a maintenance cost (aka upkeep) in gold which depends on your total number of units, and is summarized in the economic overview. The actual formula for unit maintenance is as yet unknown, and may or may not be related to the unit supply limit.

If you have no gold left and are running a deficit in excess of 5 GP per turn, units will begin to be forcibly disbanded to save on maintenance costs.

Razing a city takes one turn per population point, and begins only after you end your turn. So if you choose to raze a conquered city, you can still enter the city screen and cancel the razing. The city will then be annexed -- you cannot turn it into a puppet state anymore.

Pacts of Cooperation make the AI friendlier and more receptive to other deals. Pacts of Secrecy against another player do the same but with the added condition that neither player must cooperate with the target player.

Being allied with a city state grants you an automatic view of all tiles they can see. There is no other form of map trading; the only way to explore a major civilization's territory is to sign Open Borders and scout it with your units.

The manual once mentions that hotkeys are documented in the readme file, but they are actually listed in the manual itself on page 207. You should still have a look at the readme file ("Read Me .pdf" in your Civ5 installation folder) as it has some other useful information, such as the location of the editable Civ5 initialization file.

Looking for the real-time clock? Remember this is a Steam game -- if you have the misleadingly named "Steam Community In-Game" enabled in your Steam settings, you can simply hit Shift+Tab and get a big fat clock in the top-left corner, along with other Steam UI.

The diplomacy summary that you get from clicking the scroll in the upper right will show you all the civs and city states at once, as well as what resources they have available, how much gold they have, and how much gold per turn they are making.