A most commendable question; and you’d think a Greek dialectologist would be ideally placed to answer this.
You would be wrong. Precisely because I’m used to dialects, it’s hard for me to make aesthetic judgements on them.
But let me attempt to at least posit why certain dialects might be considered harsh.
It’s doubtful that most Greeks have ever even heard Cappadocian. Cappadocian is moribund, Cappadocians never had the numbers that Pontians had in Greece, and many Cappadocians had already shifted to Turkish before the population exchanges. So the full extent of my experience of what Cappadocian sounds like is the “Anatolian” (from Kayseri) in the 1830s dialect comedy Babel.
The Anatolian is introduced at 5:30, and yes, the very first thing he asks for is pastırma kayserili.
I’m guessing that Cappadocian will sound harsh to Greeks, because it sounds Turkish: language contact was most advanced there (even to the extent in places of vowel harmony). At an impressionistic level, it’s quite breathy, and the /a/ is back.
Then again, this recording of Pharasa [Çamlıca, Yahyalı] speakers in Greece doesn’t sound anywhere near as harsh as the music hall stereotype:
The other Anatolian dialect Pontic, by contrast, does not sound particularly harsh to me. At least, I don’t think so. Because the intonation really does soak in from the prestige language. Contrast below a newscast in Pontic in Greece (which sounds identical to Athens newscaster Greek), and a Russian Pontian speaker (who sounds, well, Russian):
EDIT: Just got this recording of Silli dialect (near Konya). Not unpleasant, but not very Greek-sounding, either:
2. Northern Greek
Northern Greek doesn’t sound particularly guttural or abrupt, but it is missing a lot of vowels. That makes it sound, at least, crunchy.
Here’s an interview with someone from Lesbos.
3. Assibilating dialects Greek
Assibilation is the process whereby non-sibilants become sibilants; in particular, tsitakismos (as the Greeks call it) is the subset of that process, whereby front [c] goes to a sibilant like [tʃ] or [tɕ]; palatalisation also made [s] go to [ʃ] in a lot of dialects.
Standard Greek doesn’t have Postalveolar consonants, and I’d fancy that any dialect that does have them would sound harsh to Standard Greek speakers.
That’s most dialects of Greek. Cretan, Cypriot, most Northern dialects, Pontic, Cappadocian, Italiot.
3. Standard Greek
Or at least, Peloponnesian, which is what Standard Greek is based on.
Ha! The rapid-fire intonation is what I have in mind. I don’t know for a fact what Standard Greek sounded like to Cretans or Cypriots when they first heard it, before they associated it with officialdom. But given how sing-song Cretan and Cypriot is, I imaging “harsh” would be one word.