What I like about it is the intuitive user interface, and what seems to be the way that applications are interacting with each other, so it seems like one system and not a bunch of independent applications, or apps as some people prefer to call them.
That's also the thing I liked with Palm/HP webOS (another OS like Meamo/Meego/Mer Core/Sailfish OS which is a Computer shrunken down successfully to pocket size, rather than some Phone middleware bloated trying to make it look "Smart").
There, the concept was called "synergy".
One one side you had a bunch of providers : you sign your Google account, and that provides contacts, e-mails, calendars, XMPP chat, Youtube video.
On the other side you had a bunch of consumer : generic e-mail application that concentrate all mail comming from all contacts (be it Google, some corporate MS-Exchange stuff, or generic IMAP), generic contacts (that automatically agregate all your address books from Google, Facebook and LinkedIn), etc.)
(For Android people:
think a little bit how some app can add a "Share with..." contextual menu on file/media/image browsers. Except the *whole* system is like this, not only the sharing/uploading function, but anything that could be provided by an account: calendars, contacts, etc.)
Compared to Maemo/Meego/MerCore/etc. webOS went even a little bit further :
- synergy was able to automatically collapse info.
if you have the same person on your SIM card's phone book, on Google, on Facebook and on LinkedIn, it is automatically detected (based on names/e-mails/Etc. similarities) and unified as a single "contact".
In Sailfish OS, that's still a manual operation (you need to "Link account..." from the pulley menu).
- Starting from webOS 2.x application could also provide "search" or "fast action" called "Just Type...".
(It's basically a descendant inherited by the universal search function on older Palm OS )
If you start to type on the hardware keyboard while no application is in focus, the system will start making auto-suggestions based on your typing
(a little bit like google's automatic suggestions, bug using above-mentionned "Just type..." plugins)
Contacts would suggest people matching your search keyword,
Notes would suggest make a new note.
if you typed a number you would be suggested to call it, if it matched a phone format, and/or if it matched a time format, calendar would propose to make a new appointment, Wikipedia and Google would suggest a search on their respective websites, etc.
That's a wonderful feature I liked a lot in webOS (and liked its Palm OS ancestror), and I miss it a lot currently.
(I've tried Google's assistant, but it is catastrophic in comparison).
And the most useful for me: all the above Linux OS (webOS, and the M*** family) have multi-processing done right.
Their card metaphore is nice (webOS deck-of-cards is even better at "tabs" and other "two-levels" problems)
Switching, starting and closing apps is only a slide or a fling away.
(Which is to be expected, as both have ancestry in desktop multi-processing, successfully shrunken down to pocket-size)
I find iOS (my friend has an iPhone) and Andoird (cheap ~100$ tablet) much more cumbersome.
Other things I find cool :
both webOS and Salfish are extremely gesture oriented.
Coming from PalmOS to webOS straigh without ever having any smartphone in between, I didn't understand what was so much the fuss about. It's only after having been exposed to Android (and iOS) I understood how much the other are cumbersome, and how navigating inside and between apps is intuitive and simple in webOS and Sailfish (everything is always litteraly 1 fling or slide away).
The most extreme difference I've seen is pickup a call between Android and Sailfish.
Android (both phone and apps like WhatsApp) seem to have a fixation about asking you to drag around on-screen icons. You to "catch" the "pick-up" green thingy, and drag it to the opposite site of the screen. You need to aim twice (picking up the thingy with your finger, and then aiming for the target zone). THEY MANAGED TO FUCK UP FITTS LAW *TWICE* IN ONE OF THE MOST COMMON USAGE OF A *PHONE* !!! DAMN!!!!!
Meanwhile on Sailfish ? They use their "pulley menu" concept. You just drag the screen up or down (no need to aim, just drag to make the "pick-up" or "hang-up" menu item appear). It's as close as the special "screen edge/under the mouse" conner case of Fitt's law as you can get on a mouse-less touchscreen. (Specially with the UI light cues, it is so intuitive)
Last webOS only fun stuff: tabs.
So basically any open "card" is a web view. Be it a website or a local application.
Handling tabs meaningfully is a non-trivial task.
On the desktop, it's more easy: you have a lot of screen estate, and you can afford putting both an application bar (KDE/GNOME/Windows taskbar, and OS X's dock) and a tab bar (tabs in firefox, in your editor, your terminal, etc.).
(You can even afford having a 3rd level: with screen showing a list of windows inside one of your terminal tabs).
On an embed device like a smartphone, with a much smaller screen, it's more complicated.
Most OSes (Android, iOS, and Sailfish) go for 2 different concept. You switch between apps and the OS metaphore is used to switch between them. And then it's the app's job to handle its sub-content (tabs).
So you end-up with that view of a vertical stack of mini-windows that you get on Safari mobile, or a grid of preview like on Firefox Mobile, or a preview thereof.
On webOS ? Well there's no tab. They'll just open a new tab. But because they have all this "deck of card" metaphor, new tabs/new sub-windows, are opened as card that are grouped together like in hands.
So when you zoom out, you see some lonely cards (e.g.: your message apps), and you see the "e-mail" stack/hand (main e-mail window with grouped inbox, hold together with other cards of each opened e-mail, and a browser card viewing a link that you're click in one message), then browser stack (a bunch of card representing a bunch of tabs), etc.
This way you can have an overview of both levels (applications and tabs/windows) when in overview/Switching mode. But this way you can also leverage all the "card shuffling" to let the user arrange them how they needs (group-ungroup cards i.e.: the equivalent of detaching/reattaching tabs, even if they don't belong to the same app) and you can use the fast/gesture switching between apps or between tabs.
And last sailfishOS terribly useful trick:
card know when they don't have full screen view (called "cover" mode) and can change their output.
- i.e.: the e-mail application won't show just a thumbnail of the full windows (like on Windows or on Android), but will show some useful info like the number of message waiting (like the "active tiles" in Windows's menu, or the various widget that you can put on the desktop in Android).
After all the above iOS and Android both seem awefully clumsy.