1. Few people care about open-source.
If you try to tell your family member or anyone who isn't a nerd that they should use Linux because it is open-source, they will not care. Even if you explain what open-source is, they still probably won't care because they sure as hell are never going to look at the source for any of it; all they will care about is that it is free. Open-source software is definitely awesome and that is a very valid force for many people, but not the majority. Most people just want to use their computer, period, and don't really want to get into the ethics of licensing and Richard Stallman and whatnot. Most people just want to check their e-mail and surf Facebook.
2. The forums aren't really that spectacular.
Yeah, this depends on the distro, but seriously, don't go out there just telling people that Linux has great support in the form of forums. This point is purely from personal experience because I'd say that at least 75% of the threads I've started on the Ubuntu forums (the forum for the largest distro out there) have either never gotten a response or were inconclusive for a fix. And it's usually not the first result when Googling. Don't get me wrong, definitely mention the forums, but just don't inflate the community's reputation. In my opinion, it would be better to lessen the chance of them not switching presently than for them to switch (you're still planting the idea), get frustrated, and never come back.
3. There is more than the community.
It is awesome that Linux is open-source software and that means that people all over the world can and do contribute to the development and it's even fine to let people know that (even if in simpler words), but don't be afraid to mention teams like Canonical, Novell, and Redhat. The misconception alot of times is that FREE = CRAPPY, mostly because they don't think free software has the resources of a big company. Tell them they're wrong. (Nicely!) Enlighten them that many of the Linux distros are backed and funded by big corporations, the only difference is that companies are committed to being user-oriented, because that's what open-source software is all about.
4. WINE is not a motivator, for the love of god.
To anyone switching from the Windows side, the appeal of being able to run their old apps in Linux is definitely a requirement. But unless you absolutely know that they run well under WINE (not just "run", "run well"), don't go throwing WINE out there. At very least, give them fair warning that WINE doesn't work perfectly or with every program. Even if you mention that WINE is a Windows emulator, most users will hear that as "You can run your Windows apps exactly the same".
1. Fight the stereotype that only nerds use Linux.
More and more, especially to distros like Ubuntu, it's becoming increasingly easy for everyday users to use Linux -maybe not on the same level as other Operating Systems out there, but still, really quite usable. Take an extra note to remind them that no, you don't have to be fluent in the command line to use Linux. (Also, if they do decide to switch, make sure to tell them that middle click pastes into terminals. From personal experience.) But you can tell this honestly; ever since switching to Linux about a month ago, I think there has been 2 things I've absolutely had to do via terminal, and they weren't activities normal users would do.
2. Be truthful. Even with the faults.
This is not a contest; you're not trying to just get one more notch on the walking stick, even if that person decides to switch back the next day. Linux is about genuinely helping people and so should your desire to help them switch. Your end goal shouldn't be to get them using Linux, it should be to get them to use and love Linux. That means including things like yes, programs like Photoshop or iTunes will not work. Yes, you may and probably will face a problem here and there. Yes, gaming on Linux is not on par with other OSes. Don't necessarily bring up Linux's shortcomings, but if they mention something like gaming on Steam or using iTunes every day, you want to be honest. Be realistic.
3. Remind them that Linux is increasing leaps and bounds.
Since they're probably used to other Operating Systems that seem to never change *coughWindowscough*, it's awesome to mention that Linux is always changing with great new features. (Also make sure to mention that these updates are optional.) But don't take it too far and act like every 6 month release is like a new OS: be realistic. Tell them that it's just evolving, at a rate -in my opinion- that is faster than ever before.
4. Bring up the programs that are cross-platform.
Firefox runs on Linux, as does Chrome, Pidgin, GIMP, Inkscape, and that is a big motivator for people thinking about switching; it was a big motivator for me, and I would consider myself more of an advanced user. But I still wanted the same programs I'd always used, and the more cross-platform programs, the more motivation. It helps to create a seamless move, even if the UI is different in KDE or GNOME or what have you. And from my own personal viewpoint, I really don't give a rats ass about alternatives most of the time. In addition to people just wanting a familiar program, some alternatives are just too different or unpolished. From my own experience, I tried every single alternative in a VM before deciding if they were actually adequate replacements. I'm not necessarily saying to never suggest alternatives, but just don't suggest it to a potential switcher like it is the same app they are used to; make it clear it is different.
5. Free Sample!
Above all, let them try it for themselves. Live CDs/DVDs are a wonderful thing. If you have the chance, sitting down with them could be great too.
Credit where credit is due: I pulled this post together from my own thoughts, but I collected the thoughts together after reading the post entitled "7 Shit Ass Reasons Why People Should Try Linux" over at Help For Linux. he makes valid points, many of which snuck into this post.
I haven't actually ever tried to suggest Linux to anybody (I need to find more nerds...), so if you have any comments of your own, feel free to make them.