Personally, I like the featureset of Discord, an IRC-like chat system designed to be better than TeamSpeak and Skype. Servers are private, and require an invite link. Server connections are persistent across multiple devices, and don't require multiple usernames for one person to connect with multiple devices at the same time. They also have their "roles" system, which gives a lot of fine-tuning to permissions—including allowing or denying access to entire chat rooms. It's useful!
But, there's a huge problem—or at least it's a huge problem in my view. They have a web app for their client, but nothing for mobile web. I mentioned this on Twitter, and the reply I got was kinda tonedeaf…
Only downside to Firefox OS: @discordapp doesn't have a responsive mobile web app to use. Need to post to their feedback page.— TeraDyne Ezeri (@Codebalion) August 30, 2016
@Codebalion We don't support mobile browsers, but we have functional mobile apps for Android/iOS devices.— Discord @ PAXWEST (@discordapp) August 30, 2016
Now, as you'll notice, I specifically mentioned Firefox OS, which I'm using on my LG Fx0 (a very lovely phone, expect a review before too long). It's not Android or iOS, so promoting the apps does nothing for me. Either the person behind Discord's Twitter account didn't know what Firefox OS is (which wouldn't surprise me), or they really weren't paying attention.
Regardless, I kinda can't use Discord on my phone, all because of an arbitrary requirement for a platform-specific mobile app. And don't get me wrong, I've used their Android app...It kinda sucked compared to their iOS app, which one of my mates uses. They don't even have feature parity with their desktop web app, much less the native desktop apps. Something tells me they don't really care about mobile. It's effectively Skype once again.
Meanwhile, there's plenty of mobile web apps that support IRC (Internet Relay Chat), and even a few IRC apps for Firefox OS (which are packaged web apps themselves). They work perfectly for the most part—though Firesea IRC seems to take a while to connect.
IRC is one of those technologies that's been around for a while, works perfectly as intended, users aren't tied to a single client. I can use LimeChat, Chatzilla (my preferred client), Mibbit, KiwiIRC (my preferred web client)…There are many IRC clients to choose from across many platforms.
Then there's chat systems like Telegram and WhatsApp…Ugh…I like the idea behind them, but the fact that a chat client requires a mobile number to simply function is annoying as hell. Add the idea of automatically adding anyone who has your number to your contacts list without your permission, and it just makes using such a service completely unappealing to me.
That's why I like XMPP. To add someone, you need to know their Jabber ID (jid), and they have to not only authorize you, but do so knowingly. It's also far more secure, since login isn't based on a finite-length standardized numbering system (phone numbers) and a login code sent to a smartphone. With XMPP, you can implement secure single-factor authentication (Kerberos, key blobs (a la SSH), etc…) or multi-factor authentication (SMS, OAuth, OTP tools, etc…)
Hell, if given enough time, I could probably adapt XMPP with some extensions (in the XEP series), take a project like Jappix, and remake the features of Telegram on my own. It's completely possible…given the time and skill.
I dunno. Maybe I'm "too old" for modern chat, as a couple of people have told me. Honestly, I prefer to see it as be being spoiled by growing up when chat systems had useful features, rather than just aesthetic features. Custom emoticons are nice, but when I wanna be left alone, let me tell people as much.