To mirror IPalindromeI's list, almost everything I do these days is centered around connectivity to modern platforms.
Writing, chatting, e-mail, and even image processing, page layout, and video editing aren't things I couldn't somehow achieve on my Quadra 840av.
The difference between 2002-2005 or so when I had my first and then second 840av, and today, is that in addition to just "doing" the tasks, I need to actually be productive while doing them.
I could write my novel in MacWritePro, but I need to be able to visit the NaNoWriMo web site and paste it in to validate it, and I would like to take a computer with me to write on -- not everywhere we go for write-ins have batteries, so I'd say one of the biggest things I miss in my oldest computers is the battery life.
In addition, like IPalindromeI, my e-mail is hosted on an Exchange 2010 server and I store my documents in a SharePoint 2010 server, which conveniently eliminates just about all possibilities of using any PowerPC Macs or excessively old versions of Office on Windows (say, if I hypothetically had an XP machine with Office 2003) to do my "productivity" tasks.
This is all connectivity issues, and to be honest there's no reason that I couldn't do this work on a good G3 or even a good '030 or '040, but I would be completely disconnected from all of the places, methods, and workflows that I use.
And, OneNote and Remote Desktop are pretty important.
To extend it a little bit -- the things I missed on the Mac for a long time were OneNote and SharePoint connectivity. Office 2011 introduced a dedicated SharePoint connection method, it's kludgy because it essentially sits next to the entire Office suite as a shim that simulates Windows' ability to connect to SharePoint. In Office 2016, the SharePoint tool is being replaced with built-in support in all the apps. Hilariously, this means that I will probably lose the ability to bulk upload unrelated filetypes to my SharePoint server from my Mac(s). It was great for putting up a large number of screenshots or photos at once.
Of course, if I did actually go back and do everything I do today on a Quadra 840av, the other thing I'd probably miss is using better digital cameras and video equipment. Though, if I had something that would put files that Premiere 3 or 4 on the 840 could open on a netatalk fileserver, that would be neat. (though, I am imagining the 840av would collapse under the weight of even something trivial to cut, such as standard def MJPEG files.)
Part of the problem is that while individually my needs are "modest" and can be done on very old hardware, either with current software or by approximating or changing some of the things about how I compute, I typically have a lot of tasks running at once. My Mac mini has, in no particular order (okay, well, in dock order) Finder, Terminal, Safari, Chrome, OneNote, Pages, Photos (doing an Aperture -> Photos batch conversion, no less), Activity Monitor, App Store, Spotify.
Despite the fact that something is pretty clearly wrong with either my OS X installation or with my mini, it chews through all of these things(1) and doesn't flinch even I leave them open and launch a game like World of Warcraft.
And none of that's even considering virtualization, which I can happily do a lot of on something like my now-years-old Sandy Bridge desktop. (the main advantage there is dual hard disks and sixteen gigs of RAM.)
Raw horsepower is part of it, and Intel-based computers have that in spades. A modern overall architecture is the other component and although it's broken in Mac OS X, the ideas that it uses are solid and have served every other modern desktop operating system very well.
(1) My ThinkPad T400 will happily do the same workload with no random stalling out, even though it is half as fast and has half the memory, so my tasks aren't necessarily something I currently need a brand new computer or even a high end one to do.
