first, when one removes the paper, the film left behind is still part of the paper itself. Its kind of felty in texture. This HAS to come off also, usually soap and water will remove this quite easily. It sometimes will come off with a wet wash rag.
Usually this is from only vinyl papers.
If this does not come off, then when you paint it, the stuff will eventually release and take the paint with it.
Second part, semi gloss is becoming quite the choice of wall paint, especially when kids are around or in areas where cleaning of the walls are necessary.
Semi gloss is very cleanable and will withstand the abrasive action. BUT flat paint will actually come off and will not clean at all.
One texture is flat and shiny and the other is bumpy and rough.
Semi gloss is not as "plasticy" as emanal or high gloss.