That huge multi-gig download and install of Xcode is one reason I had for leaving OS X for linux. The hurdle for getting FOSS software was too big.
Almost all of the tools I need I can install in minutes in Debian with apt-get. It's reliable, easy and gets me doing work instead of crying while I open 20 tabs trying to find solutions to obscure bugs on Google.
Although, the en vogue thing to do is to use a Linux in a VM. You get to keep OS X and all it's friendliness and indispensable proprietary programs, and you get to have a full Linux dev environment that's also easy to backup and "snapshot".
(Now, if the commercial software for OS X, from iWork to Photoshop, and from XCode to pro audio/video drivers, is not "indispensable" to you, by all means, use Linux).
Not necessarily just one though, they could be 2-3 proprietary options that have absolutely nothing competitive in OSS land.
(If people use Wine for some of them, its beside the point: that only re-inforces they do find them indispensable. Not to mention that very few pros use Wine for their work, and that Wine doesn't work 100% glitch free).
As for what programs I think. Well,
• Photoshop is one of them, sure. GIMP misses a lot still that professional graphic designers need and use everyday, especially in it's CMYK workflow. Same for Illustrator.
• InDesign (or Quark for some) is also indispensable. The only OSS competitor, Scribus, is not even close.
• Programs to design and edit fonts. Nothing with the breadth/ease of the few commercial options.
• Logic/Cubase/Sonar/Live/Pro Tools are also indispensable. Nothing comparable to OSS. Same for the multitude of pro quality plugins, from Waves to UAD.
• Premiere/Avid/FCP(X) etc. Video in OSS land is a royal pain, and nothing of the short and extend of those programs exist.
So, graphic design/dtp/video/music/sound (anything media/multimedia in short) is a no go. And I think not only in the high end, but also in the everyday consumer space. GIMP does't have the ease of use of Photoshop Elements. Nor is there something as powerful but simple as Premiere Elements or iMovie. Something as simple as editing your daughter's birthday video and adding one two effects and titles can be a world of pain. Tons of other things, from big to small, like CD labeling or card-making software with hundreds of templates to pick.
Lots of other things I guess, but as they are not exactly in my workflow, I don't know about them. Is there Mathematica/Matlab for example? or Autocad? What's the state of 3D modelers? etc.
however, I doubt people would balk at 5gb on a laptop install as most people don't really have space limitations there.
I decided this in 2005, when a multi-GB download and 4GB install was relatively larger. And no it's not just a one-time thing, I had multiple laptops and I did reinstall them (adding linux to try out etc).