Good grammar is important. I have often seen a brochure, catalogue or other material being very well presented but totally ruined at the last minute by bad punctuation, spelling mistakes and so on. Most of the world might not notice, but to the few who do, the offending errors stick out like ' a vast bowl of pus' to quote Monty Python. Lynne Truss' "Eats, Shoots and leaves" is an excellent book for dealing with punctuation, though at time it plods on rather ponderously. But it's not just a matter of 'being correct and propah' because, as Lynne's book prefectly illustrates, a cavalier attitude to syntax, grammar etc., can result in radically different meanings. Consider -

"They could have not wasted all the money" V. "They couldn't have wasted all the money"

See any difference? (I hope so)

But apart from that, if you are talking about creative writing, many other factors come into play. I feel creative writing should a) say something new, or in a new way b) enagage your attention   c) have beautifully crafted prose - economy of word, fluidity of line

Obvious examples include Thomas Paine's "Rights of Man" and "Cuomo vs. O'Connor" by Ellen McCormack (for how to present an argument systematically and devastatingly), Lord of the Rings by Tolkien (for how to develop an imaginary world in such consistent detail that it stops all disbelief), "The Machine Gunners" by Robert Westall (for accurately portraying an adolescent's experience of wartime UK, and on being an adolescent dealing with monolithic parents, school bullies and a grown-up world and politics).

I have just finished reading "The Life of Pi" by Yann Martel. I literally could not put it down. The prose is sublime. The insights astonishing. I have read it while queuing at the bank, while my customers weren't looking, at the breakfast table, on the public transport till my eyes hurt.