A book is considered a classic when it stands the test of time and appeals to generation after generation of readers and is relevant to all of them.

As a classic book, Great Expectations, contains vivid characters who struggle for survival, discover love, encounter failure, work hard and achieve success.  Dickens fills his novel with real experiences filled with alienation, lonliness, ambition, success, failure, family, and self-discovery. Pip grows up in this novel and his journey is very relevant to the human experience. 

"However, modern critics have little but praise for Dickens' brilliant development of timeless themes: fear and fun, loneliness and luck, classism and social justice, humiliation and honor."

I haven't come across many other novels that are as dense and meaningful as Dickens's Great Expectations.  While I've had students moan and groan at the prospect of having to read it (I think mostly because they've heard it's long and know that they'll HAVE to read it), they generally come to understand just how masterfully Dickens is able to give us insight into human interaction and the human spirit.  (Take, for example, the open-ended essay on the AP exam; Great Expectations is the third most frequently listed novel, because it examines so many different aspects of both intra- and inter-personal relationships. 

Meaningful plots and superb characters make classics.  Few novels ever are designated as classics without intriguing characters, and Dickens is an absolute master at creating memorable characters.  For, who has read A Tale of Two Cities ever forgets Madame Defarge, who has read or seen "A Christmas Carol" ever forgets Scrooge?  And, who has read Great Expectations and does not remember the innocent child who is ridiculed for no reason--who has not experienced this same gratuitous cruelty? And, who ever forgets the eccentric Miss Havisham?  How bizarre, yet understandable, is she?

In response to #3, I absolutely agree with you.  The canon is definitely judged by these standards, so there are many that aren't in the canon that should be, in my opinion.  I'm very glad you brought this up because it is a very, very good point!