What am I? Answer punctually

Question

I have not graced many eyes.
Usually I mean surprise.
I'm shaped adjoined,
in a contest I was coined,
and I never got my chance to rise.

I might just be a line and dot,
and I might be a lack of thought,
and though some might scream,
I'm a journalist's dream
when he goes to sleep on his cot.

This story ends with a bang.
Down under, we've marked up the plane,
flight 8253. You could add seat 203D.
And my name is the ________.

Hint 1:

The title is also a clue.

Hint 2:

Yes, the word does indeed complete the rhyme.

Hint 3:

One line is two clues, and two lines are one clue.

Hint 4:

Given the "adjoined" clue, think about what it means when its parts are separated.

Answer 1

Is it...

the interrobang‽

I have not graced many eyes.

It's a fairly rare punctuation mark.

Usually I mean surprise.

The interrobang represents disbelief and shock.

I'm shaped adjoined,

It's two punctuation marks joined together.

in a contest I was coined,

I can't find any evidence for this - it seems to have been coined for an ad campaign rather than a contest.

and I never got my chance to rise.

It's still very uncommon.

I might just be a line and dot,

It's made up of a line and a dot.

and I might be a lack of thought,

It could be seen as "lazy" to fuse two punctuation marks together.

and though some might scream,

It can represent a strong outburst of emotion, like a scream.

I'm a journalist's dream when he goes to sleep on his cot.

Maybe journalists rarely see them, but want to see them more often? I don't quite get this part.

This story ends with a bang.

"Interrobang" ends with "bang".

Down under, we've marked up the plane, flight 8253. You could add seat 203D.

8253 is the decimal Unicode codepoint for the interrobang; 203D is the hexadecimal version of the same number.

And my name is the ________.

Interrobang!

Title:

It's a punctuation mark.

I'm not quite sure that rhymes, though. :P

Answer 2

At this point, I'm gonna go ahead and answer this one. I apologize if some clues were too confusing. As mentioned by Mike and Deusovi, the answer is indeed the interrobang, a symbol that look like a ? superimposed on a !, like so: ‽. Keep that in mind.

I have not graced many eyes: The symbol is fairly rare, such that most people probably haven't seen it.
Usually I mean surprise: The symbol is to indicate surprise. Most people separate the two symbols and end their surprised statement with a !?.
I'm shaped adjoined: It's a ? and ! adjoined together.
In a contest I was coined: Or at least its name was.
And I never got my chance to rise: Another reference to its rarity.

I might just be a line and dot: That's literally what it is.
And I might be a lack of thought: This was the line everyone seemed to struggle with. As I mentioned above, it's a contraction of !?, which is a chess annotator's symbol. As explained in that Wikipedia article, it's been referred to as the "symbol of the lazy annotator who finds a move interesting but cannot be bothered to work out whether it is good or bad."
And though some might scream: Another reference to its usage.
I'm a journalist's dream / when he goes to sleep on his cot: Speckter, the head of an advertising agency, ran the aforementioned contest to name the Interrobang. He came up with the idea because he thought ads would look better with one punctuation point instead of two. Thus, it's a "journalist's dream" - specifically Speckter's. The part about sleeping on his cot was added for slight humor and to finish the rhyme.

This story ends with a bang: Get it? Interrobang?
Down under, we've marked up the plane, / flight 8253: These lines make up two clues, not following the line break. "Down under" refers to Australia, and specifically the State Library of New South Wales, whose logo is - you guessed it - the Interrobang. The second clue was "marked up...8253" - that is, HTML character ‽.
You could add seat 203D: "You...add...203D" - Unicode character U+203D.
And my name is the ________: Do I really need to explain this?

Hint 1: The title is also a clue: Pun on "punctual"/"punctuate."
Hint 2: Yes, the word does indeed complete the rhyme: Reference to the very last line of the limerick.
Hint 3: One line is two clues, and two lines are one clue: Reference to "Down under, we've marked up the plane, / flight 8253." The first line is split into two clues, the second half of which combines with the following line to complete that clue.
Hint 4: Given the "adjoined" clue, think about what it means when its parts are separated: Reference to the "lack of thought" line, in which the chess annotation was given because of splitting the symbol into its parts.