TeX/LaTeX is great, I use it in many ways. Some of it's advantages are:

• it uses text files, this way the input-files can be diffed and many tools exist to work with text
• it is very flexible
• it has a stable layout: if I change something at the start of the document, it doesn't affect other things at the end of the document
• it has many extensions to reach different goals (a successor would start without extensions, but would have a good extension-system)
• you can use standard build control tools to support complicated documents (thanks dmckee)
• you can encapsulate solutions and copy&paste them to new documents or send them to others to learn from (thanks dmckee)

But on the other hand some little things are not so good:

• it is hard to learn at the beginning
• it is complicated to control position of images
• a few things are a little counter-intuitive
• sometimes you have to type too much (begin{itemize} ... \end{itemize})

So, does there exist a successor/alternative to LaTeX or at least is some hot candidate for an alternative in development. A real successor/good alternative would keep the advantages and fix the disadvantages, or at least some of them.