Well, these are all really good questions...
I try to answer them to you.
What makes it superior?
Better support of different machines. Basically no memory restrictions.
Some of the most used features already implemented. Bugs of the original
Wolf (known ones) has been removed and the original code has been improved
(for instance drawing walls/items)... Well, there are more reasons aswell
but if memory is the most important.
There was one ToS-feature that was impossible to append due the famous
"Abnormal Program Termination" in original DOS-source can now be added as
SDL pushes the limits way farther.
What new coding features would it allow one to do?
I believe advanced shading (like in Doom) would be easier to add now
(tough the_fish released something similar for Wolf3D), also expanding
maps to 128*128 could be done without much tinkering.
And support for larger palettes and such...
Oh, and thanks to AlumiuN - SDL version supports now MIDI,IT,OGG,MP3
music and higher than 8bit wavs.
Think it as in this way. Feature you have seen in RoTT, Duke or Doom
can be now added without taking away some other features.
Can I convert my existing mod/source code to SDL?
Well, not directly - depends on what features you have been using.
Though most of the major coding features people wanted for their DOS mods
has already been ported to SDL (tutorials can be found from SDL code crakkers
in Diehard Wolfers)...
So to sum this up... SDL gives you an illusion of a total freedom to add
nearly anything you want to the Wolfenstein (if you can code it). Of course
SDL has it restrictions too - but to face them, you must have made some sort
of megamod with incredible amount of stuff.