Curious to know how people interpret the comment on its duration.

"It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it." 

Would this mean that the spell cant even be cast in areas of moderate wind? As this is a concentration spell i would assume that as long as the caster maintains concentration the fog is maintained, regardless of the wind/air flow. Because why have it as a concentration spell if wind could just get rid of it?

While the spell does indeed requires concentration, that doesn't mean the effect remains no matter what. The spell requiring concentration means that if the caster loses concentration, for one reason or another, the spell ends and the fog dissipates. However, spells such as Gust of Wind or Warding Wind could still disperse the fog and end the spell, in the same way that the webs conjured by a Web spell can be burnt away, or how the darkness conjured by the Darkness spell can be lit up by effects such as the Daylight spell.

Can this spell be cast underwater? Marids have this spell & an encounter I have planned is completely underwater.

This spell didn't work well in my campaign so I'm swapping it for something else. I even had the blindfighting fighting style. It always caused problems for the party and they were always annoyed when I cast it. Also, suddenly an enemy caster had blindsight as well, making the point of the spell, well, pointless.

Can creatures outside the fog see into the fog cloud?  As in, to shoot arrows or spells at those within the fog?

I've been trying since last night when it came up in a game to find out what happens with Fog Cloud and Wild Magic.  Does Fog Cloud get cast at level 1, or at the same level as the spell which was used that caused the wild magic?

So level 2 spell is cast, it triggers wild magic, so is the size of the fog 20 ft radius or 40 ft radius?

Remember, this is a level 1 spell, so something as mundanes like a strong wind dissipating the cloud isnt unusual. Darkness is a level 2 spell and has less radius as well scaling options, but requires higher level spell to get rid of. 

Also yes, creatures shooting INTO the fog cloud from outside have disadvantage and have to pick a square they are shooting at as well..  the creatures inside have full concealment

Fog Cloud doesn't have to be casted on the ground, so you could create somewhat of a cover from sunlight above the area you fight in. Which makes this an optimal spell for Drows and Kobolds.

Like the Bow/Crossbow using Kobold Ranger could concentrate on that spell to shield himself from the sun while making use of Pack Tactics advantage.

Quote from theredwind >>

Also yes, creatures shooting INTO the fog cloud from outside have disadvantage and have to pick a square they are shooting at as well..  the creatures inside have full concealment

You only have to guess what square a creature is in if the creature you are shooting at is also hidden such as using stealth. There is nothing about the blinded condition that causes you not to know where an enemy is located. If the target is merely not visible to you, but can still be heard or detected by some other means, then you would already know its location and could attack it at disadvantage as described in the PHB rules on Unseen Attackers and Targets.

Drop one of these bad boys when surrounded by enemies that have a lot of advantage. Wolves and kobolds come to mind with their pack tactics.

The "At Higher Levels." is not Italicized! IT'S NOT ITALICIZED!!!

I once used it to convince a group of NPCs that a bar was on fire. Once they cleared out me and the bois looted it.

Guys, don't forget the errata about vision and light: 

• General: A heavily obscured area–such as darkness, opaque fog, or dense foliage–blocks vision entirely. A creature effectively suffers from the blinded condition (see appendix PH-A) when trying to see something in that area.
• Errata: Vision and Light (p. 183). A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.

In other words, those who are inside may have advantage against those outside the spell range (up to the DM though), however if both creatures are inside, they have advantage and disadvantage against each other, thus effectively canceling each other's out.

This spell completely shuts down creatures that only have truesight (and that don't have blindsight). Since they won't be able to see through the fog.

Which makes upcasting this against a Lich completely worth it. So many of their spells and even one of its abilities rely on being able to see their target. Including spells like power word kill, disintegrate, finger of death, etc. Sure the reverse is true for your party but the lich will very quickly die if it is unable to remove the fog. They can just dispel it but that takes both their action and potentially (if this is upcast) a higher level slot. They could counterspell it but the range of this and its ability to go around corners means that is easy to avoid. Just cast it from behind a pillar or wall.

Advantage and disadvantage also cancel out when you are fighting in fog (because you can't see them and they can't see you). So your party really is still very likely to be able to wack away at them pretty effectively.

the real MVP

Would this spell block sunlight from a source like the Holy Symbol of Ravenkind?

I'm a late Comer to the discussion, but would this block the dragon's blind sense as well?

Nope.

Blindsense - if you are able to hear, you are aware of the location of any hidden or invisible creature within 10 feet of you.

They'd be able to hear anything within 10.

Quite the boring spell if I do say so myself, but very useful.

Thanks!