> I remember X-Symbol, but most of its feature set has been replaced by
   > other things (e.g. input-methods, or \inputenc{utf8} which lets you use

Do you mean \usepackage[utf8]{inputenc}

   > actual accented chars in the file).  Are you referring to the replacement
   > of "\alpha" with an "á" and things like that?

Do you mean (set-input-method "TeX")? 

Last time I looked into that, my impression was this functionality
included less symbols than x-symbols. Besides sub and super indices were
not supported.[1]

I really did not get it to work well in a LaTeX file: either compilation
failed or the symbols were displayed quite ugly in the dvi/pdf file.

   > These could be added via prettify-symbols-mode (which, like
   > preview-latex relies on redisplay features rather than modifying the
   > actual buffer's content).

I am curious to see whether that would have performance issues on big
files

Uwe 



Footnotes:
[1]  personal taste: the symbols provided by the input method look a lot
     uglier that the one provided x-symbol