NOTHING SO NEEDS REFORMING AS OTHER PEOPLE'S HABITS.
I don't understand the use of SO in this sentence. What does it refer to? How can I rephrase the sentence? Can anybody come up with another commonly used expression with this NOTHING-SO-VERB?

   Hi Bruno.

   Welcome to the world of the language of literature, having a bumpy ride, huh?

   The sentence coined by Samuel Langhorne Clemens, whose pen name Mark Twain.

    He began writting in 1865, so the well known phrase might have been said sometime between before the year of 1910 (year of his death). 

    What I mean is, we have to bear in mind this is long time ago, the choosing of words were somewhat way different of today´s speaking. 

     That´s more, it have to be weighed the literary of Twain, and the guy was quite a traveled, educated one.  No to mention, he was well into the society of the time.

    http://en.wikipedia.org/wiki/Mark_Twain

   In modern standard English it would be rephrased to "nothing needs more reform than other people´s habits/ nothing needs more reform than the habits of other people (than the habits of others an so on and on...)"

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =  

    In fact, nothin so + verb is not very commom today, so you can take it that is

an example of "literatism" of that  age.

    On the other hand (sorry for straying from the subject a bit), even today "nothing so" can be the shortening for "nothing IS so...", there is, sometime you can use both.

http://www.happiness-project.com/happiness_project/2011/01/nothing-so-fatiguing-as-the-eternal-hanging-on-of-an-uncompleted-task.html

"Nothing So Fatiguing as the Eternal Hanging On of an Uncompleted Task."

"Nothing [is] so fatiguing as the eternal hanging on of an uncompleted task."
--William James

 (see what I mean? in the same site the guy use the two ways of writting.)

Nothing so flash as modesty    (could be "Nothing is so flash as modesty!" )
http://www.brisbanetimes.com.au/opinion/society-and-culture/nothing-so-flash-as-modesty-20111010-1lhid.html