"In my opinion it is necessary that a rock situation be set up in front of the house so that the floodwaters of the river will be prevented from coming into the house and ruining the private property of the people that live in it.

But it is impossible to remember the drunken face of McCarthy, merry often with a kind of worldly malice, as if he were mocking those who took him seriously, and believe that he himself could take seriously anything but his boozed-up nightmares."

--Lillian Hellman, Scoundrel Time, p. 37

Both of these sentences are grammatically correct, but the first one seems blanked out, its author numb and mechanized, setting the words down to get them over with. In Hellman's sentence, by contrast, we feel that the writer is interested, involved, energized by what she says. 

Her sentence is long, but it is unified; the words "drunken" and "boozed-up" give it a head and a tail, and the tail has a sting. So it isn't just grammar which marks the difference between a good and a bad sentence; it's the author's sense of making an act, getting something across - a message, an image, an order - and of not trailing along behind it, but leading it forward.

   Sentence Fragments
   Comma Splices
   Run-on Sentences
   Dangling or Misplaced Modifiers
   Subject-Verb Agreement
   Split Infinitives
   Which, That, It, and Of
   Semi-colons and Colons
   Apostrophes
   His and Her