Storage space is never something I've required a lot of, but this is especially true of the storage space on my computer. I don't keep a lot of files; I have a few gigabytes of photos, tens of gigabytes of music and a hundred or so gigabytes of digital video, and not an awful lot of files otherwise. The most numerous files other than music and photos are my blog post drafts, there are hundreds of them, but they're all stored in plain text and take up no space at all. I have only a handful of applications installed, some of which came with my MacBook, and some others that came with my MacBook have been removed (not counting the myriad little tools and utilities that go largely unused on any computer). On my iOS devices I perhaps have double that number, but many of those fulfil tasks for which OS X has bundled software. The largest single application in terms of drive space is The Elder Scrolls Online, my current time-waste of choice.
Out of the 256Gb solid state drive in my MacBook I'm currently using less than 100Gb, and I suspect that when I nuke and pave the drive to upgrade from the Yosemite beta to Yosemite proper this figure will drop a little more. There's a certain amount of stuff on here (such as Xcode) that I won't need in the release version. You'd be correct in pointing out however that my SSD usage and previously mentioned media ownership don't match up. My movies and music are stored in 'the cloud' and streamed on demand. I don't need to keep local copies (but I can download local copies should I need to). Come early 2015 the same will be true of my photo library, but photos will still require local copies of at least part of my library. It remains to be seen how much space that will save. The primary benefit of cloud storage in this case isn't necessarily the saving in local storage, but the centralised accessibility. I am of course accessing that media from a number of devices. Just because the space saving is secondary it doesn't mean that it isn't valuable.
My general documents live either in iCloud Drive or in DropBox, neither of which save any disk space but both of which make sure my documents are available to me on whatever device I have to hand and are probably worth mentioning. I also have a Nifty Mini Drive, which is a neat way to add additional storage (especially for a secondary backup location as micro SD cards are virtually indestructible).
As for my limited number of apps, I try to use the apps that I have before installing replacements. This means that I'll try and use Apple's preinstalled software before looking for other alternatives. This doesn't work for everyone, especially people that need especially feature rich tools. I tend to only need the most obvious functionality for an application, such as e-mail, for example. In the majority of cases I haven't needed to go hunting for more powerful solutions. There are cases where the standard apps haven't been as useful to me as third party alternatives, I use Byword rather than TextEdit, for example. I've discussed the apps I love before, though this list is now a little out of date.
It's certainly true that I'm not typical in my storage needs. Probably the majority of heavy computer users will have more expansive media collections, more personal documentation and the like. Almost any other self-identified gamer will have far more than a single game installed. I'm pretty strict with myself, though I'd call it realistic; I can only give so much of my time to playing computer games, so I pick the game that I most want to invest that time in and remove others that may distract from that. I simply don't need the million-and-one features of the more powerful e-mail clients. I tend to make an app earn it's place on my hard drive; or at least make myself justify it's installation by making use of as many of it's features as possible. If an app's features are going largely unused then I probably don't need it.
I am living proof that even the nerdiest of us (I'm a software engineer after all) don't need terabyte after terabyte of disk space, though!