alanabit, I totally agree that "... one characteristic of good lyrics is that they are easier to learn. They are memorable, have clear ideas and original rhymes."

Actually, I think Dylan has written a few well-known 'turkeys' if you judge that by rambling, unedited lyrics.  He has some very tight, well-formed songs like Blowin' In The Wind, I Shall Be Released, You Ain't Goin' Nowhere, and One Too Many Mornings, but he also has some unnecessarily wordy songs with hard to remember lyrics such as Masters Of War, Love Is Just A Four-Letter Word, Boots Of Spanish Leather, and Mr. Tambourine Man.  Not that the latter four are terrible songs, but they are, IMNSHO, not as tight, lyrically, as the former three.  (I think he may have been stoned or having a tantrum when he wrote some of his songs.)

I entirely agree with alanabit that good lyrics, with stronger imagery are easier to learn. Several of  Paul Simons's songs have lyrics that are quite memorable and easy to recall because, even if they are "nonsense," they have been carefully tooled to flow just right with the music (and conjure up images).
 

McGrath, I don't know why one should assume that songwriting as such would not improve with practice, the study of music theory, or exposure to more and more music.  Most art forms and other endeavors do.

A few songwriters whose work I really amire are:
Kate Wolf  -- for her marveous ability to paint wonderful word pictures that are also, usually, meaningful and, always well matched to the music
Bill Staines  -- much the same excellent marriage of words (sound and meaning) to the music (though I don't think his imagery matches Kate's)
Sting  -- part of what I admire is his versatility in style
Carole King -- partly because her songs sound so different from each other  (similar to what I said about Sting)
Paul Simon -- can sometimes make a profound point with words (Sounds Of Silence), other times incorporate incredible whimsy (Fifty Ways To Leave Your Lover, Me And Julio), and occasionally just really move me with the marriage of words and music (Bridge Over Troubled Water).

Both Paul Simon and Randy Newman (and of course the incomparable Tom Lehrer) have that marveous gift of humor and whimsy in their lyrics.  And John Prine just does wonderful things with words -- even though I don't always know what he means.  Theirs is a kind of talent hard to emulate.  On the other hand, I am also in awe of the lyrical genius of Cole Porter, Irving Berlin, Johnny Mercer, Hoagy Carmichael, Ira Gershwin, and several other songwriters of the Jazz Era in the US, and I think their type of genius can, to some extent, be nurtured and developed-- and applied to various musical styles, including folk music.

I've heard it said that "good writers" say that a cardinal rule is to write every day -- no "if"s, "and"s, or "but"s.   Maybe part of the reason why there seems to be a lot of crap peddled today as "songwriting" is that too many "songwriters" take Hank Williams's attitude but have not the natural genius to be able to put together a gem in 20 minutes.   If you look at other endeavors, such as dancing, gymnastics, figure skating, opera singing, science, etc., you seldom find anyone outstanding who has not spent countless hours doing tedious excercise, study, practice, etc.   Why on earth would one EXPECT to be great at songwriting without anything of the sort?   (Yes, Handel said that "The Messiah" was, essentially, given to him by inspiration, and the tune to "Yesterday" came to Paul McCartney sort of in a dream, but these are the exceptions, not the rule.)

The point is, I think, if it takes Irving Berlin's writing 1500 songs to come up with about 15 American standards, why does that make him less an artist?  (I don't think the monkeys could come close in 15, 000,000, 000 songs,)

For me, FWIW, "Yesterday" is a nearly perfect song.  By that I mean that:
--the tune is so natural that I, like McCartney when it first came to him, could swear I'd heard it somewhere before,
-- the words fit perfectly, naturally with the tune/melodic rhythm, without having to be squeezed or stretched into place.
--the lyrics are easy, conversational sentences, not contrived  -- yet they are not clichés, either.
--the rhyme scheme is perfect -- again, without seeming to have been manipulated
In short, I cannot imagine changing anything about the song -- tune, meter, words -- that could make it a better song.  It has the unpretentious elegance of a rose.
Yet this song is not one that came to the writer fully formed.  Paul thought of the tune first, then played around with silly lyrics like "Scrambled eggs, oh, my darling, how I love your legs...," and after WORKING on the song, finally arrived at the finished project.

Interesting, though, Shambles, that my favorite songwriters tend to write songs that could stand on their own as poems, even if you did not hear the melodies.  They also tend to write songs that grab you even when sung by mediocre singers with mediocre instrumental accompaniment.