Calling someone “a transgender” is sort of like calling someone “a gay” – it’s not a noun (not even a plural noun, like “gays”).
Even with “transsexual,” which is a noun and an adjective, many people prefer to be referred to as transsexual persons, since it emphasizes their personhood.
My own personal baggage makes the distinction for me. When I was a kid, we were one of a small handful of Jewish families in an overwhelmingly Christian suburb. And yes, there was anti-Semitism. There were kids at school who routinely yelled “Hey Jew!” or “Hey you . . . Jew!” (or “Christ killer!”) at us, and even after more than 50 years, the word makes me uncomfortable.
And in my case, I’m not even sure in what sense the word “Jewish” applies to me. I’ve been an atheist since the age of 13, and am not even that Jewish culturally. But Judaism is my heritage; I have enormous respect for what my ancestors went through, but I don’t share their beliefs. And please don’t call me “non-practicing;” that implies that I still believe in the religion, but don’t practice any rituals. That’s not me.
And linguistically . . . I like the way “Jewish” has a sort of non-absolute, relativistic feel to it, like referring to a color as “bluish.” You can be sort of a Jew, in some respects, but not entirely.
And there’s a definite parallel between “Jew” and “homosexual.” I prefer “homosexual” as an adjective, not a noun. It *describes *what I am, but doesn’t *define *me.
Possibly also the anti-semantics. 
This reminds me of an ep of H:LotS. I cannot remember the ep name, but it’s from season one. Belzer is one of the cops investigating a beating death, and he’s interviewing a father of a suspect. The guy has a creepy white-supremacist vibe and he’s almost circling Belzer.
Dad: You’re a Jew, aren’t you?*
Belzer: I’m Jewish, yes. <very calmly deadpan>
Dad: I could tell.
That’s the difference. Sometimes the “a Jew” sounds really creepy.
I’m not Jewish, not anything -ist, -ian either.
*This is from memory. I haven’t watched my DVD’s in a while. YMMV. Caveat emptor. etc 
Wow. I was totally unaware of this. But, then again, I was, until recently, unaware of someone’s Jewishness being something that was discriminated against. Sure, I’d heard of anti-Semitism, but I thought it was something that happened a long time ago. I still have never heard anyone ever use Jew pejoritively, and probably have trouble understanding them if they were referring to some sort of stereotype.
I was similarly unaware of anything about gypsies, and was unaware that being “gypped” had anything to do with prejudice. Same with “going Dutch.” I thought it was a different word–the similarity to the nationality was coincidental.
Have I mentioned that I’ve led a sheltered life?
Nearly 30 years ago, Danny Kaye was in a TV movie about Nazis marching in Skokie, Ill. At one point, someone in authority calls Kaye and his associates “Jewish people.”
Kaye responds, “You can say ‘Jew.’ It’s not a dirty word.”
That exchange is quoted from memory so I’ve probably got it completely wrong, but it made an impression on me and I made it a point never to treat ‘Jew’ like a dirty word. It saddens me a little that even some Jews seem to regard it as one.
Maybe I shouldn’t have listened to Danny Kaye, but now it seems unnatural to say “Jewish people” in some contexts: “The committee included Christians, Muslims, and Jewish people”?
I agree, it’s all about context and tone. “The Jews returning to their homeland” isn’t bad, but “Hey, you filthy Jew” obviously is. It just depends on the circumstances as Dio said.
In my own experience (before I became a Jew), I knew that the word “Jew” wasn’t bad, but I tried to use it less than “Jewish” or “Jewish person.” It just seemed harder and less friendly for some reason. Interestingly enough, now that I am a Jew I don’t have any particular feeling when I use “Jew.”
So?
So the grammatically improper truncation of the adjective from “Democratic” to “Democrat” is done specifically to avoid attributing the descriptive quality of the word and to denigrate the party name.
I’ve never noticed the “Democratic” description, but I’ve been tuning politics out lately.
But the fact that they’re doing this must mean that someone thinks that “Democrat” is a pejorative (<-Yes, that is the correct spelling…you learn something new every day).
Wait, wait… so if someone calls me “a Spaniard” I’m supposed to take offense?
This controversy sounds real strange to me.
Someone calling you “a Spanish” would be more analogous, especially if there was some kind of pejorative history with it.
I know your English is basically perfect, so hopefuly you can pick up on the subtlety there.
It’s not like “a Jew” is horrifically offensive the way “kike,” or “nigger,” or “spic” would be, by the way. As I said before, it’s more crass (in some tones or contexts) than patently vicious.
How about “A Brit?” or even “A Swede?” I just don’t see the outrage inherent in calling someone “A noun” rather than an “adjective.”
OTOH…
This I agree with. Which in my, personal mind means we should work to *erase *the pejorative history rather than to *perpetuate *it – light a candle rather than curse the darkness, as it were.
ETA: And I agree with Dio (and with others above) that, in the end, it’s mostly in how something is said rather than in the exact words that are said, anyway.
And I still think the more pressing problem is that we need two words – one for the religion, and one for the demographic.
Grammatically speaking, that would be like calling me “a Jewish.”
Agreed.
Yeah, that’s a pain in the ass. It’s hard enough getting people to distinguish between “from India” and “Hindu,” in your case the notion that someone can be “ethnically not Jewish but religously Jewish” or “ethnically Jewish but religiously (insert religion or lack thereof here)” just makes too many people’s subtlety meters explode.
I just wrote a paper on “the Clash of Civilizations,” so I have “pigeonholing” even more in my brain than I usually do.
I think looking at it as a strictly grammatical question is misleading. It’s problematic because of its context in the history of antisemitism, not (only) because of its conjugation. I think the example of “Jap” is a good one.
So the problem would be people thinking that “Jew,” instead of a complete word, is a derogatory abreviation? Which would link to the “I’d like you to use the complete word” in the OP.
Yes, but I was making the comparison to “a homosexual,” which is what I thought you were doing with “Spaniard.”
I think it would be closer to calling a black person a Negro. Sure it’s as accurate as calling someone ‘a Caucasian’ (though when does anyone get called that, outside of academic papers or studies, rather than ‘a Caucasian person’?), and sure it’s pretty arbitrary as an insult, but try saying it aloud. ‘Two Negroes and three Jewesses attended the party thrown by the gays.’ It sounds like something out of an old textbook that’s go a chapter denying evolution.
I think the take-home message is that none of these words is really grammatically or semantically analogous to another, and you have to work out what each one means and how to use and avoid using it according to the context.