On The Street(s)

[CaseyA]

Suppose while outside of any enclosed areas (home, houses, buildings), I met someone. Would I write

I met someone on the street.

or

I met someone on the streets.

?

[Tdol]

I would say in the street. (BrE)

[CaseyA]

http://www.nytimes.com/2011/02/20/we...pagewanted=all

Egypt’s military, calculating that it was no longer worth defending an 82-year-old, out-of-touch pharaoh with no palatable successor and no convincing plan for Egypt’s future, ultimately sided with the protesters on the street, at least for Act 1.

http://www.nytimes.com/2011/02/23/wo.../23cables.html

That was then. Today the young protesters on the streets are demanding the ouster of the entire family, and it was Seif el-Qaddafi who declared on television at 1 a.m. Monday that Libya faced civil war and “rivers of blood” if the people did not rally around his father.

There must be some significant differences between "on the street" and "on the streets".

[Rover_KE]

There's no difference.

They both mean 'in public, open-air urban areas'.

On another occasion the respective journalists may well choose to add or omit the s.

Rover

[5jj]

Egypt’s military, calculating that it was no longer worth defending an 82-year-old, out-of-touch pharaoh with no palatable successor and no convincing plan for Egypt’s future, ultimately sided with the protesters on the street, at least for Act 1.
Today the young protesters on the streets are demanding the ouster of the entire family, and it was Seif el-Qaddafi who declared on television at 1 a.m. Monday that Libya faced civil war and “rivers of blood” if the people did not rally around his father.
There must be some significant differences between "on the street" and "on the streets".

In those two, there is no significant difference, in my opinion.

In your first example" I met someone on the street (s)", the singular is more acceptable. It suggests simply that you met someone while you were walking.The plral suggests to me that the person you met was walking the streets aimlessly, or even living rough. Like Tdol, I would use the BrE 'in'

[Tdol]

There's a difference between meeting someone in one street and a large protest which could occupy a number of streets- I would talk of protesters on the street(s), unless, say, they were directly outside my house and my focus was purely on them being in that place.