If you want to browse THE internet safely, learn safe practices; Linux has nothing to do with safe browsing.
Limited game support for Mac - One word: Blizzard. Screw any other games.
Limited community support for Linux. Games? What are those? We are programmers, see my printf();. :unsure:
Limited credibility for Windows. Well, I have no reason to get SP3. SP2 works for me; not much into gaming (except for the soon to be released StarCraft 2, and GTA4).
When can you not upgrade a Mac at any point in time? Sure you can: Video cards, Memory, Drives, Displays -- Apple uses the HDMI stuff now --, I am pretty sure inputs, being USB2 based can be "upgraded," among other things. Want to upgrade the processor? Sure, unplug the old one, pop the new one in... Wait, you cannot upgrade it! Get a new motherboard, memory, video card; while you are at it, get a new display, drives; oh! ooh, I can run xxxx now -- get a new display -- Hell, why not just get a new Mac... Please, Mac's are upgradeable; when you upgrade a non-Mac system, what the fuck do you think you are doing? Putting in a new motherboard, because the features in your new operating system require a certain type of processor (maybe 64 bit, perhaps) which requires a new motherboard -- and while you are at it, might as well get a new set of sticks, plus try out the Raid Array you've always wanted. The new onboard sound is not that bad, so you won't upgrade the sound.
Seriously, do some homework before making random claims, dude.