There are bound to be some limitations on IE8.
File Upload is definitely one area where you may see more differences than most - no upload progress bar, possibly limited to 1 file, all those sorts of things.
Obviously the client likes this sort of restriction (and they like the web to run slow) or they would upgrade IE (or, if on XP, install a browser that is currently supported like FF or Chrome.) From a web-app developer (the emphasis being on app, not site) I don't get too concerned about their choice of browser, and I don't get bogged down by IE8. I kinda give them it's the "it's your choice" speech, and I don't get involved in browser wars. If they want to run an old, out of date, browser then they are completely free to do so. That decision has consequences (for them, not for me.)
Incidentally if they are stuck on IE 8 because of some legacy system, then they're free to install Chrome or Firefox and have 2 browsers on the machine. Legacy stuff doesn't mean that _everything_ on the machine has to be bad.
From a NetTalk point of view I don't go out of my way to break IE8 - but I don't go out of my way to support it either. Given that it is slower, and supports less of the HTML5 spec, it's understandable that many things I may want to do won't work in IE8. Incidentally jQuery 2.x has dropped support for IE6,7 and 8 and although they are running parallel development tracks at the moment (1.9.x does still support them) at some point that track will drop and we'll naturally move to jQuery 2.
So if anything, IE8 compatibility will get worse, not better. Like I say, it's their choice....
Cheers
Bruce