I can't answer specifically for guinea pigs, but here's what some googling found.
First and foremost, if humans don't generally eat it, there must be a reason. The fact that it doesn't kill you with the first few bites don't necessarily mean that it's not harmful in the long run.
Second, what doesn't affect us as humans may well affect guinea pigs. Our bodies are large enough to tolerate the effects of eating a few bites of something that's not particularly good for us, while theirs are not.
Third, you have to be very careful of what the leaves and plants may have been sprayed with. Although it's not a perfect system, there is some protection against some pesticides for foods that are in the grocery store, but not so for things that are found in the wild.
And after reading all this stuff, I have to say that with the possible exception of raspberry leaves, I wouldn't feed any of that to my pigs. Is there some problem with just feeding them common vegetables, like lettuce, bell peppers, zucchini, tomatoes, etc?
Google results:
Raspberry leaves are edible in general -- the younger the leaf, the better.
Milkweed is only edible after cooking, because it has a very bitter taste and must be boiled to be tolerable. However, several sites caution that milkweed is easily confused with dogbane and butterfly weed, both of which are poisonous.
Blueberry leaves are edible but have medicinal effects, which would be unknown in guinea pigs.
Grape leaves are edible.
