It's really easy to define a term such as 'slavery' sufficently, even if it has to include a vast variety of practices. The problem is not the definition, but to have people agree on it. Is wage slavery really slavery? Some people would say yes, others would say no and both would be sure to have it right.
Even if you try to bring forth a 'minimal definition', with which everyone should be able to agree that everything that falls under it certainly is slavery, there will be problems, when you look at it closely. Things like forced penal work or legal guardianship are hard to differentiate from slavery and people will here, again, disagree on what is a form of slavery and what not.
Also, the slavery debate is rarely one taking place on the field of reason, but rather on the emotional plane. Definitions can't really do justice to the emotional side of things.
All that said, I think it'd be a good thing to agree on a definition for the sake of the discussion. Even if one doesn't agree with all facets of the definition, it helps making clear where one disagrees and where one agrees and generally gives a discussion a structure that allows to better understand what is actually talked about and how.