First off, the biggest security threat is social engineering.
There are idiots - sorry, no better way to describe them - using phones on all the major platforms. From the earliest cellphones capable of polyphonic ringtones it was "I want a free ringtone", and now with smartphones it's "I want a free app". Of course you also have your music, television and movie torrents as well. There is generally no limit on what you can find for *free. Any operating system can be insecure. Any mobile device can be broken into. The ease of access depends on the method being used. Social engineering, for example, can result in a user voluntarily surrendering their login credentials. Two factor authentication helps, but what happens when your recovery questions can be solved by looking at your public Facebook profile?
End of soapbox...
Why is Windows Phone good for first-time smartphone owners? For starters, it's a relatively lag-free user experience. It doesn't take much processing power to push a dozen or so live tiles on the start screen. Immediate shortcuts to applications, contacts, and phone functions make using a Windows Phone less daunting. There is a seamless, polished merger of social networks into the People app. Multiple email accounts can be merged into one unified mailbox. Internet Explorer renders pages properly, if the website adheres to actual standards. The app store is simple to navigate and applications that do not work for your device are excluded from search results. If applications do not work for your device, the reason(s) why are noted.
Nokia/Microsoft devices feature a suite of user-friendly, accessible applications that provide the user with a rich experience. Nokia optics on flagships like the 1020 and 1520 produce fantastic imagery. The best camera is the one that you have with you, and these devices prove that. For the low cost of *free, Nokia has/had provided users with impressive camera controls, maps and navigation, a curated app store with featured apps, and unprecedented support after devices are released.
Unlike Apple, there are multiple devices that run the same operating system. Unlike Android, the end-user experience is the same. If you provide a loved one with a Windows Phone, you can easily troubleshoot the device over email, Skype, or on the phone. There aren't modified versions of Windows that are skinned by Samsung, HTC, or LG. And while there is "bloat" - preinstalled applications from the carriers - it can be uninstalled. Uninstalling these apps doesn't prevent you from getting system updates. You don't have to jailbreak or root your phone to accomplish this.
Every Lumia device I have owned - the 900, 920, 1020, and now 635 - have functioned extremely well as a phone. Extremely well. That's something you cannot necessarily say about other devices on competing platforms. This is subjective, I know, and every year companies seem to get increasingly closer to providing a phone that is smart but still works as a phone.
Expandable memory....
On Windows Phone you can install applications, music, and video on a microSD card to free up space on your device. Yes, there are devices that run Windows Phone and have no expandable memory. However, Apple doesn't offer devices with expandable memory. You have to buy a device with higher storage capacity if you want more space. A completely new phone. Android limits app installation to the main memory, but with 5.0 Lollipop it appears that they are relinquishing that hold on external storage. Developers have to program for that, though.
I worked in retail for several years before getting out of business to consumer and moving to business to business sales. I enjoyed working in electronics / video games. I often found myself pushing Nintendo consoles on families with small children because it was a more family-friendly investment. If people likes sports titles, I suggested the Playstation. Granted, there were exceptions to this rule. I would deviate from this technology bias when I wanted to push a Nintendo 64 onto a hardcore gamer, or get a dad with a 10 year-old to buy a Playstation so they could try Gran Turismo 2 multi-player.
People can be technology savvy but have no idea how to manage a smartphone. My father-in-law is an engineer and has problems with his television, computer, and iPhone on a regular basis. Meanwhile, my wife will find herself saying "I know you can do this, but how?" and researching online or asking me questions to find the solution. He enjoys an iPhone and she prefers Android. Would either of them enjoy Windows Phone? Sure.
Sometimes you need to step away from your technology bias and just listen to what you or someone you are helping actually need. Do you use iTunes? Do you have $100's in paid applications from the iOS app store? You probably have a good investment in Apple already so the iPhone is a solid choice. Maybe you are service neutral, and don't have a significant investment in one company (Apple, Google, Microsoft) compared to the other. Maybe Android will work? It's open. How about iPhone - Google apps are available. (As are Yahoo!, Microsoft, etc.) Then you have Windows Phone. You can still do all that. Let's not forget about Blackberry, either. You can run Android apps on those devices.
My opinion has always been this - shop for the network, not the phone. If you pick a phone because it's your everything and the network was secondary, you are probably going to complain about a lack of signal in the areas that matter most. The same rule applies to Windows Phone/Android/iOS. If you want iOS, your option is limited to iPhone. However, you are assured to have a device that will last for years with regular updates. Just don't drop it! Android offers more options, but the lifespan is significantly reduced. User experience is not consistent between devices and hardware/software limitations become evident with each new OS release. Fragmentation is a problem. Windows Phone offers users choices, but with a consistent user experience. With 7.0 and 7.5 devices at the tail end of that generation just before the launch of Windows Phone 8, there was noticeable fragmentation. (Windows 7.8, anyone?)
I think people should have choice - choice in handsets, choice in operating systems. That isn't going to happen anytime soon. Meaning, you couldn't walk out of the Microsoft store with a Lumia 1020 and later decide to forgo Windows Phone for Android with a system update. You couldn't put Windows Phone on a Galaxy S5. iOS won't run on an HTC One. It's a shame. So until a proposition like that comes to fruition, you are left to buy into one ecosystem over another. You can go all in (Apple, Google, Microsoft) or have a mix of the three. Obviously the best experiences for Apple are with the iPhone, Google on Android devices, and Microsoft services run better on Windows Phones. You still have that iOS/Android/Windows Phone wrapper around your experience, regardless of what route you take.
Windows Phone is a good first choice for a new smartphone user, the jaded smartphone user looking for something different, the budget conscious user that wants to incorporate modern technology into their workflow, and the business person that wants to keep work and home life separate. Side note: I'm coming from a year of iOS and brieft stints on a OnePlus, LG G3, and a Nexus 5. I picked up a Lumia 635 on Monday. It's probably the best device I've owned in the last year.