Phizzy's note:
"A good number of scholars in the field, I would venture to say most, do not think the "John" who wrote the "Revelation" was the same John who wrote the Gospel of that name, command of the Greek language, as mentioned above, being one big clue."
A great deal to that. Even as early as Eusebius.
From “History of the Church” by Eusebius of Caesarea ~ 340 AD
Book III Chapter 25. The Divine Scriptures that are accepted and those that are not
Eusebius examines arguments pro and con for several disputed books of the Bible, including entrants that are not included today. He quotes the detailed arguments of Dionysius on Revelations or the Apocalypse of St. John and they are much the same as those debated today. Howe much earlier his source spoke of this matter, I am not able to say. I don't think Eusebius weighed in either way on Revelations, acting here at least as an historian of the dispute rather than the arbiter. Luther also had "reservations about Revelations" being in his German translation of the Bible. That he could wave about translating this book suggests that the notion of canon in the 16th century was not as mature as we might be led to think. On the other hand, the 19th century Anglican annotated Greek Gospel of St. John explores the same issues as Eusebius and more or less shrugs them off.
With my own limited knowledge of Greek, I have simply compared the Gospel John's language with that of the John of Patmos in the NT Concordance of the Greek. The latter certainly talked a lot more about "mouths", "beasts" and "swords" than the former did. But even in the 4th century discussion, the writer of Revelations was taken to task for his Greek vs. that of the author of the Gospel attributed to John.
Here is an excerpt from Eusebius.
“But the Apocalypse is different from these writings and foreign to them; not touching, nor in the least bordering upon them; almost, so to speak, without even a syllable in common with them. Nay more, the Epistle, for I pass by the Gospel, does not mention nor does it contain any intimation of the Apocalypse, nor does the Apocalypse of the Epistle. But Paul, in his epistles, gives some indication of his revelations, though he has not written them out by themselves.
"Moreover, it can also be shown that the, diction of the Gospel and Epistle differs from that of the Apocalypse. For they were written not only without error as regards to the Greek language, but also with elegance in their expression, in their reasonings, and in their entire structure. They are far indeed from betraying any barbarism or solecism, or any vulgarism whatever. For the writer had, as it seems, both the requisites of discourse, that is, the gift of knowledge and the gift of expression, as the Lord had bestowed them both upon him. I do not deny that the other writer saw a revelation and received knowledge and prophecy. I perceive, however, that his dialect and language are not accurate Greek, but that he uses barbarous idioms, and, in some places, solecisms [violations of grammar, incongruities]. It is unnecessary to point these out here, for I would not have any one think that I have said these things in a spirit of ridicule, for I have said what I have only with the purpose of showing dearly the difference between the writings."
-End of Eusebius excerpt.