While there is a wide range of opinion among adult-baptism protestants as to whether a Catholic (or other infant) baptism is or can be considered valid, there seems to be (at least in my experience) a general attitude that Catholic communion and marriage are considered valid by protestants. That is to say, if a protestant were to visit a Catholic church and participate in the Eucharist (communion), most protestants would not have a problem with this (whether the Catholic church would allow this is another question). And certainly if a couple got married in the Catholic church, and later became protestant, no protestant church (that I'm aware of) would require a re-marriage (then again, if a couple gets married before they are Christian at all, I've never heard of any protestant churches requiring a re-marriage in this case, either).
However, if a protestant couple gets married, then later becomes Catholic, they must be re-married in the Catholic church (or at least this is my understanding!). And Catholics are not permitted (by the Catholic church) to participate in a non-Catholic communion (with a few possible exceptions?). I don't know if Catholics recognize a protestant baptism, or require re-baptism when a protestant becomes a Catholic. (Related question here)
So my question is: On what grounds does the Catholic church reject communion, marriage, and (possibly) baptism of protestants?