How unique, culturally, were the Thracians from Classical Greeks (i.e. roughly 480-338 BCE)? They seem to have an outsider status in the texts, but I’ve never seen any examples of them being hugely different than the Greeks in terms of culture or archaeology—admittedly haven’t read into it much. Did they speak another language, or was it their culture that made them gain this status among the Greeks? Or could it have been the intensity of Greek xenophobia?
Before I start this is my first post on this sub so I hope it can stay! I'm only an undergrad in archaeology, but I have done field work in Bulgaria and am currently doing research on Thracians.
Some parts of your question are easy to answer and some are not so easy.
The word "barbarian" comes from the Greek "bárbaros" and means "to babble". They called nearly all foreigners barbarians, including Persians and Egyptians, who we would not apply the term "barbarian" to, given our modern understanding of it. So we can't simply apply our modern understanding of the word to the Thracians and understand why they were called that.
The Thracians did speak their own language, which we now call Thracian and linguists believe it is an Indo-European language, but there are very few known words from Thracian, and are known mostly by being loanwords into other nearby languages, or being attested by non-Thracian sources. The Thracians themselves had no written language or literary culture, and left only four known inscriptions, which were written in Greek script and don't tell us very much. So not so much is known of their culture, except what we can tell from archaeological contexts and foreign writers (upon whom we can place only so much trust).
So simply by being non-Greeks, they were barbarians. This may, however, still leave the question "were Thracians considered savages and why?"
Not having a literary culture would contribute to such an identity. From what we know, they lived primarily in smaller village communities and had few major cities, Seuthopolis being the only known polis of the Thracians (and it's sunken under a reservoir now, so little else may be learned of it). They were also known to be very fierce warriors, and Herodotus wrote of them “if they were under one ruler, or united, they would, in my judgment, be invincible, and the strongest nation on earth”. Xenophon wrote that Thracian soldiers served as mercenaries under the Spartan general Derkylidas in the 3rd century BCE, and it's known that they were involved as mercenaries in a number of other conflicts in Greece. This may have some influence on their perception, but it also may be that our modern understanding of "barbarian" colors the way we look at them.
For further reading, I highly recommend the excellent Blackwell companion "A companion to antique Thrace" edited by Julia Valeva et al.