FileVaultMac OS X lets you encrypt the contents of your hard disk (under System Preferences > Security & Privacy > FileVault) which was something I wanted to do on my previous laptop. Unfortunately, I waited until I had about 150 GBs of stuff on the old 200 GB hard drive, so much that there wasn't enough room to encrypt it all & the process would have taken forever to complete in any case. With my Air, one of the first things I did was enable FileVault. This makes it more difficult for just anyone to boot from a flash drive & read the contents of my hard drive.
Cloud StorageOther than a few vital items I keep in encrypted disk images on my hard drive, I've pushed almost all my files into four services: Google Docs (now Drive, with added storage space), Google Music, Spideroak, & Dropbox. I use Google Docs for presentations, spreadsheets, & text. Being able to access my documents from anywhere is a serious boon. Google Music keeps my massive music collection from clogging up my hard drive & has a nice web app for my phone as an added bonus. Dropbox is great for web development & contains most of my git repositories. I'm nowhere near using up my free storage space, because most of my files are plain text (code, documents, notes).
One issue with Dropbox is that, while I can choose which directories sync to my hard drive, I cannot choose where those files end up: they always reside the Dropbox directory. That's where Spideroak comes in: Spideroak lets me backup files without moving them, which is essential in many cases. For instance, I'm syncing the settings for Sublime Text across three devices, settings which have to live in the Application Support directory. Furthermore, I trust Spideroak far more. Dropbox had one of the worst security failures in recent memory, while Spideroak has a very reassuring "know nothing" model.
Moving to the Cloud is more difficult (or at least impossible to do for free) for someone with more media. I have virtually no video files & few images, so I'm able to use free storage quotas with plenty of spare room. The items I care about most are either code or text, both of which are extremely lightweight.
ApplicationsOn my previous laptop, I went overboard testing out different web browsers & code editors. With my Air, I'm trying hard to only install programs I know I use, rather than slowly piling up junk which I use once & then never return to. Honestly, storage is not the concern, I just don't like the idea of having unused detritus lying around, clogging up my QuickSilver catalog.
This goal also works well with the Cloud: why install a word processor when there's Google Docs? I keep thinking I'll need Libre Office at some point but that day hasn't arrived yet. For many tasks, not only is there a suitable web app, but my favorite application is the web one, e.g. Gmail, Google Docs, & Google Calendar. I actually wish Apple would give up on packaging iCal, Mail, & some of their other defaults because they get in the way more often than not.
RegretOne decision still troubles me: would I be better off with Ubuntu, Debian, or another GNU/Linux distribution? It's not simply that open source software aligns with my ideals & is easier to customize than Apple & Microsoft's fare, but that dependence is never a worry. I can find everything I need on Linux & then I don't have to worry as much about format migrations (remember when .docx was such a pain?), expensive updates, & lock-in.
At the moment, I'm content with Mac OS X. There are still a few standout applications (most notably Quicksilver, but many of web design apps tend to cater to the Mac) & I do enjoy the look & feel of OS X. That said, Lion is a distinct step backwards from Snow Leopard in a few areas: Launchpad & Mission Control are both worthless to me (because I launch apps with Quicksilver) but they replaced the very feature that made me love OS X in Exposé. While there are new touch gestures available, I must admit that I haven't found a use for any of them yet. So that's disappointing & I'm positive Apple will continue in this direction, slowly absorbing OS X into iOS in the very worst way possible.