I'm sort of skeptical that any of these will be usable as a complete replacement for a very long time. Heck, I can't even use pdflatex today because the toolchains for several conferences and journals (I'm looking at you, Journal of Functional Programming!) require both the output to have been emitted by the latex (chokes on the PDF otherwise) and for the document sources that you provide them to also be compileable with it.

I started using ConTeXt (http://wiki.contextgarden.net/Main_Page) after many years of LaTeX. The command interfaces are more homogeneous than with LaTeX and it is quite easy to do complex layout with it.

ConTeXt is an awesome package. Its "only" drawback is that it's unsupported by publishers. I thought about writing my thesis in ConTeXt, but then I couldn't have easily reused the text from published papers.

Tangentially related: How does one write 'modern' LaTeX today? What packages/tools do people use to ensure their documents work properly?

Looking at the compile script for my CV, I'm currently using latex, bibtex, and xelatex, along with sed to work around some nasty bibliography implementation issues.

LaTeX really needs to be just revamped whole. I haven't been following the development of LuaTeX or LaTeX3, but I hope that's what they both do. The syntax needs to be simplified, concerns that no longer exist thanks to improved hardware (such as super-optimized algorithms for hyphenation) need to be discarded, and all of the popular 3rd party packages need to be integrated into a single, cohesive piece of software.

What are the problems with Liang's algorithm?

On a modern computer you could easily store the entire dictionary, but it's not clear that that's actually an improvement since it wouldn't be as good at dealing with unknown words.

Have you looked at ConTeXt?

http://wiki.contextgarden.net/What_is_ConTeXt

In our research group, almost everybody just uses emacs to write and aspell to check.

One person also uses TeXLive, but he is the one non-emacs person for development as well (uses bbedit, I believe).

TeX Live or MikTex (Windows) can get you started; they include the standard compilers, dependencies, and tex2pdf-like scripts.

I guess both new LaTeX3 syntax and the Lua part will be hidden if I'm a casual user. So I will indirectly benefit from both, while the package and class authors will hopefully use the new capabilities.

And why not working with a LuaLaTeX3 - the best of both worlds can be used.