Math isn't the problem if you are happy with the "normal" math fonts used already by pdftex. This will work fine with xelatex + lualatex too. You can also try unicode-math but I don't know if it works in all cases.
The multilanguage support is more problematic: As you are using different scripts (greek, russian) you can't use babel (at least for this languages), as it will break the unicode font support. So you need polyglossia and this doesn't work with lualatex yet as it use (at least for some languages) xetex specific commands like \XeTeXinterclass. Also the support files of some of the languages (e.g. french) are much more sophisticated in the babel version. It is possible to mix babel + polyglossia but it depends a lot on the actual language combination if and how good it works.
Regarding the microtype support: The newest version of xetex can do protrusion (I haven't tried it yet), lualatex can protrusion + expansion. The author of microtype has just announced on c.t.t. that a preliminary version of microtype exists which supports both engines.
But at least for lualatex it isn't needed, one can activate both without problems manually:
\documentclass[fontsize=12pt]{scrartcl}
\pdfprotrudechars1
\pdfadjustspacing1
\usepackage{lipsum}
\usepackage{fontspec}
\newfontfeature{Microtype}{protrusion=default;expansion=default;}
\setmainfont[Microtype,Ligatures=TeX]{Linux Libertine O}
\begin{document}
\lipsum
\end{document}
I can't speak for XeTeX, although I am very sure that both LuaTeX and XeTeX can load metafont fonts. So the answer to your question is "yes". But if there is a Type1 or OpenType alternative, I'd always go for that.
LuaTeX loads cmr10 by default, not latin modern. Of course one could change that, but the idea is to get as much portability between the different engines as possible. So if you run a document through PDFTeX and through LuaTeX, both results should be the same.
The user is supposed to use the fonts he or she wants. Computer Modern und thus Latin Modern has the huge advantage to be a) free b) a big set of fonts (italic, monospaced, sans serif,...) and distributed with all ancient TeX systems. If the distributions chose another font as the default font, nothing much will be gained as there is not really much to do more than:
\documentclass{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\begin{document}
Hello world.
\end{document}
While in principle I like your idea, I think it is extremely unlikely to ever happen, so I would not bother to think too much about it.
Best Answer
The only difference is for Linux: OpenType or TrueType fonts from the TeX directory structure, e.g. the texgyre fonts, have to be assigned by its filename for
XeTeX, e.g.\fontspec{texgyre-pagella.otf}. LuaTeX itself also searches the TeX font directories, the reason why LuaTeX finds such font defined by its family name, e.g.\fontspec{TexGyre-Pagella}. For MiKTeX it should make no difference, fonts should be found.The package
mathspecworks only forXeTeX. Andpolyglossiadoesn't work with LuaLaTeX yet.