To use your example of Mormons and Catholics, the theological difference is vast--Catholics (and all 'traditional' Christians), for instance, view God as self-existent, eternal, infinite, and omnipotent. Mormons believe that the personage we call "God the Father" was once a being of flesh and blood, and is not eternal, infinite, or omnipotent, and that humans can "progress to Godhood"--"as man is, God once was, as God is, man may become". Pentecostals are far closer to Catholics on many theological issues than Catholics and Mormons.
I had, as an aside, several meetings with Mormon missionaries this last summer, where we discussed these very issues, and looked over relevant Mormon and Biblical texts on the subject. It was a fascinating series of discussions.