General:
Both pages let a person espress a feeling by venting without requiring much thought. It provides a generic template of "personal" feelings, and makes  us realize just how anything one wants to say these days will just sound like a "line."
they are limiting inthat they offer specific choices in the fields, but those choices are what make the pages unique in that it's all there for you, just cut and paste. A better way to do it would be to allow the writer to put in their own phrases wherever they see fit and not just in a paragraph seperate from the given choices and sentences.
A page that makes more serious letters could also be a workable idea. We are in the age of speed and we want condolence letters and sincere apologies that sound deep and touching, yet are generated on the fly. It would save us all having to go through the emotions involved(apathy, anyone?).  Then come the moral implications which you may discuss with yourself later because they're boring.
They follow the letter format well: greeting, subject, conclusion, signature.
 

Specific:
Apology letter:
Good: It gets mailed to the person intended and refuses to go unless it has a return address on it.  You can opt not to send it.
          It has some pretty decent phrase choices that come pretty close to expressing actual remorse
          It is funny if it has to be and has some witty remarks
          Short
Bad:   Sometimes we are not given the option of a serious phrase, or one that is sarcastic enough, so it is hard to maintain a certain tone.
           All the reasons the writer is apologizing for are house chores, which is really annoying

Stalker letter:
Good: Interesting idea
          good letter format
Bad:   limited phrases, lame humour.
          Messages cannnot get sent but instead are read and then if the person who runs it likes them, she will post them on the page. Otherwise, forget ever seeing it.
          Sounds like the person who wrote it got dumped and wants to sound like she ahs all these admirers, pretty sad.