\left(\int some stuff \\
some stuff on a new line\right)
and have the parentheses, like, resize the way they would without the line-break.I don't think that Markdown is a viable replacement for LaTeX. The benefits of LaTeX over TeX are convenient font management, cross references, bibliographic databases, etc (although, if you can do without "convenient font management", TeX with eplain will get you most of what you want LaTeX for). The benefits of TeX are page layout and typography. Neither of these really fit Markdown. TeX on its own isn't much heavier than Markdown.
It would be interesting if some of these lightweight alternatives --- pandoc, org-mode, etc --- would target TeX and handle the LaTeX functionality themselves. It would obviously be more work, but it might lead to something self-contained and distinct.
I think the original author's points have merit: for example, the error messages are horrid, and in my case it's certainly not helped by adding another layer of indirection. But, I also fully agree with you: it's still better than everything else!
LaTeX is not only very extensible, but also, in my experience, very stable. Even typesetting hundreds of pages filled with figures (both rasterized images and vector graphics made with TikZ, which is wonderful by the way), equations, tables, references and stuff, just works.
I do agree on the error messages though. However, so far a search on tex.sx have solved everything. Sure, spending 30 minutes on a bug only to discover a misplaced closing bracket sucks, but to me, that's a tradeoff I can live with.
If anything should ever replace (La)TeX, my bet would be something like Markdown. However, I think it'll take a while... LaTeX has a HUGE community and a large array of packages written for all sorts of features.