Regarding "do the needful", Wikipedia has an article on the subject. It indicates that it was more common in English in the past. I don't think it is grammatically wrong; it is just more a matter of idiom in US/UK English. There we would more likely say "do what is necessary" or "do whatever it takes".
The same is true with pluck the flowers. It is grammatically fine; it is just not the idiom.
In regards to prepone, this is an Indian coining, and I personally think it is a great word. However, it has not made its way to Europe and the US. Frankly, I think it is our loss.
Indian English is a perfectly legitimate dialect of English and need in no way feel inferior to the mother country's version. On the contrary, India has been an abundant supplier of words and phrases to British English, and we owe the Indians a debt of gratitude in that respect. "Pluck the flowers" might be a little odd sounding to the British or American ear, but Americans "could care less", while Brits "couldn't care less", and Americans don't get too pissed, and Brits don't get too pissed off at each other about the differences.
As we English speakers like to say: vive la différence.
I think with careful speakers/writers, explicitly structurally adjectival magical is usually reserved for metaphoric usages, whereas magic tends to be more literally to do with the "supernatural". So if I were considering the utterances of a careful speaker, I would expect this distinction...
1: "You should read this magic book" (it's a book about magic)
2: "You should read this magical book" (reading it will be a metaphorically enchanting experience)
Note that where the referent is something that often occurs with the "noun-used-as-adjective" form (such as magic trick, lamp, beans, spell, wand, etc.) it usually carries a sense that the target noun has supernatural powers. My careful writer could imply that his book actually had supernatural powers in speech (by placing very heavy stress on the word magic), but in writing it would require rephrasing or help from context to disambiguate that third possible sense.
I suspect that (particularly in casual speech) there may be a tendency to use the shorter adjectival form more often, simply because the "more appropriate" full version sounds a bit highfalutin.
Having said that, there are plenty of established collocations where no literal reference to the supernatural is implied by the short form (magic square, lantern), and some that almost defy categorisation (magic marker, bullet). Those are the ones you just have to learn to get used to.
Best Answer
Neither is wrong. They just mean different things.
“What memory!” refers to "memory" as a general concept.
If someone remembered things very well, you might say "What memory!" to mean "What (good) memory (you have)!"
“What a memory!” refers to a specific memory.
If someone told you a story, you might say "What a memory!" to mean "What a (good) memory (you have just talked about)!"
Edit: As noted in the comments, you could say "What a memory!" to refer to the general concept of memory as well. The example would be similar to "What a (good) memory (you have)!"