I remember my first real job doing Web stuff. I was working Working for Verizon SuperPages, and had to build our sites using this template based CMS. We would find ways to hack the CMS by putting code in places that weren't design for it, and customizing sites. This was 2001 - 2005. It was pretty much an IE world. But even then, we an issue came up I would check my sites on a Mac we had up there that was running IE5. What a mess. I remember being so relieved when the Mac/IE5 browser feel off the map. Little did I know that this was only the beginning.
The next layer of IE hell was IE6. Because of Windows Vista being crap, people stuck with XP for a long, long time (me being one of them). And because of that, people stuck with IE6 a long, long time. IE6 didn't really prompt you to upgrade. Then, when all the cool HTML5/CSS3 stuff started coming out, IE users were left in the dark. Even when IE9 came out, which was somewhat HTML5/CSS3 compatable, its usage was pretty low, because of all the XP machines still out there that couldn't upgrade to IE9 even if they wanted to. Most XP machines were 32bit and IE9 ran on 64bit systems, which was mostly Win Vista, and people were slow get rid of XP. There for a while you could buy PCs with either OS, and lots were still choosing XP (me being one of them).
So now IE6 is pretty much out of the picture, although I just had a project last week that required IE6 comparability (now where did I put that PNG hack?). But IE7-8 are really not much better than 6. Yes, I can use transparent PNGs, but there are all kinds of rendering issues, as well as little to no HTML5/CSS3 support. Granted, IE9-10 have a pretty big numbers, but some really cool stuff just doesn't work in IE7-8 and does not degrade gracefully.
This is where all the hacks, patches, etc are still needed. Luckily frameworks like jQuery have a lot of built in IE fixes, but you still need Resond.js, Modernizr, html5shiv, etc. to fix some specific issues. And those issues can turn your cool-ass web site/app in to crap if you don't find an IE solution.
This is a very worn-out old rant that many have made over and over again. It just still comes up so often that I have to purge from time to time.
The good news is that if all browsers were equal there would be a big "make website" button somewhere that would crap out a site, and my work and pay would substantially decrease.