As most of us know, the Taiwanese language and the local language of southern Fujian (Minnanhua) are almost identical. In Chinese class - I'll post a nice long rant about that later, suffice it to say that I'm a bit fed up with the Mandarin Training Center - my teacher consistently calls Taiwanese "Minnanhua" and everywhere you turn people will testify that they are the same language.
Fair enough. For most purposes, they are. But I posit that "for most purposes" is not quite as comprehensive as it should be, and that Taiwanese is a dialect of Minnanhua...related to it, and more or less the same language, but not exactly. Rather like Indian English, which is, of course, "English" - but who reading this has been to India and can testify that Indian English is sounding the same as American English, only? People are speaking it quite differently isn't it? If you want to be communicating pukka Indian English, of course you have to be changing-changing a few leetle things, only, so it is not same. So then you are having New Yark people or all people in USA speaking in one kind of English only and Mumbai desi speaking other kind.
Now I'm no linguist, but perhaps someday I'd like to be (to be posted forthwith on my rant about Shi-da, the reasons why I'm equally likely to get an MA in Linguistics as I am in Chinese at this point). I don't even speak Taiwanese well...just a few phrases here and there, and the names of lots of food, because food is the ultimate bonding agent between new friends. Food and alcohol. The Chinese and Taiwanese understand this, hence the term jiu-rou pengyou ("liquor-meat friends" - friends who are more 'buddies' that you go drinking with, but wouldn't necessarily confide in).
Even someone like me, however, with a very limited knowledge of, well, stuff, can point out loads of differences between Dai-gi and Minnanhua. The first is the most obvious - if the accent of Taiwanese speakers as well as the slang used changes between Taipei and Kaohsiung (and it does - my Taiwanese-speaking friends from both cities have confirmed this), then how can one possibly expect it to be the same between Taiwan and southern Fujian? From student reports, the difference is minimal - the difference, maybe, between American English and British English. Some words and phrases are different and the accent is quite varied but they're generally mutually understandable.
The next point is a little less obvious, but still quite true: Taiwanese has a plethora of vocabulary that is quite different from that of Minnanhua. Huge swaths of even the most basic vocabulary for everyday communication is completely different. By completely different, I mean from an entirely different language. The most basic subsets of Dai-gi vocabulary do not come from Fujian; they come from Japan.
Here's just a short list of the ones I've discovered so far (some of which were brought to my attention by my friend Joseph, whose Taiwanese is better than mine...for now):
Dai-fu - doctor
Sian-si / Sian-sei - Teacher (from "sensei", which one student says came from "xiansheng"...?)
neku tai'u - necktie, from English via Japanese
hinoki - redwood, clearly from Japanese...the Japanese loved consuming the cypress tree resources of Taiwan
obasan - an older woman, also from Japanese
I could see the argument that having a few vocabulary words borrowed from Japanese makes no difference in the relatedness of Taiwanese and southern Fujianese...and indeed in terms of basic structure or language family lineage, it doesn't. Just like English isn't any more closely related to French than it was when it evolved simply because we borrowed a bunch of words from it.
However, I'd also note that English is a decidedly different language from a lot of languages spoken across northern Europe (most strikingly, Icelandic, which apparently is closer to Old English than the English we speak now is) and one for that is the sheer weight of word borrowings from other languages. There's also the fact that the words that come from Japanese seem to be the most basic vocabulary - we're not talking high-falutin' words or 'elite' or 'literary' terms (the way Korean borrows a lot of Chinese), we're talking words like "doctor", "teacher" and "old lady".
Seems to me that this makes a pretty good case for Minnanhua and Dai-gi to be dialects, not identical languages. But like I said, I'm no Linguist.