What isn't spiritual communion? Spiritual communion is a prayer one can pray if they can't receive the Eucharist at Mass and it's a good prayer to pray if you're making a visit to a tabernacle outside of Mass.  Sacramental communion can be conferred in extraordinary circumstances outside of Mass, and I would think that the same graces conferred through reception of the Body and Blood of Christ during Mass would be conferred outside of the Mass setting. (Is conferred the right word?)

But what graces are not conferred on the pray-er who asks for spiritual communion?  Or, less specifically, what is found in sacramental communion that is not found in spiritual communion?
According to the Council of Trent Catechism for Parish Priests,


  those who, having it in their power to receive with fitting preparation the Sacrament of the body of the Lord, are yet satisfied with a spiritual Communion only, deprive themselves of the greatest and most heavenly advantages.


The text states that the first grace required to receive the Eucharist (sacramental communion) is "given to none unless they receive in wish and desire" (spiritual communion). Spiritual communion recalls the soul to life, giving the grace required to receive "the fountain of all graces", sacramental communion. The text states that language is inadequate to fully describe the fruits of the Eucharist itself, but gives as examples:


feeds, invigorates, and delights the soul
unites the soul with the substance of the sacrament, i.e., Christ
remits venial sins
strengthens against temptation by displacing the lusts of the flesh with the love of charity
in short, facilitates the attainment of eternal life