Even as a traditional UI, the iPad isn't exactly a shining example of proper UI design. Before we go any further, I think it is important to emphasise that I'm from what is now regarded as the old school when it comes to user interfaces - I believe in consistency, predictability, and logic. It should be obvious what things do, they should be in the same place and do the same things in different applications, and there should be some sort of overarching sense of direction, purpose - logic, if you will - tying it all together.
The reason I'm telling you this is to create context. I'm the kind of person who still gets worked up every time I launch Google Chrome - that's how much I value the things I mentioned above. Current operating systems have been clobbering these staples of good UI design to death for years already, but none of them go to the kind of lengths the iPad does.
iOS on the iPad is inconsistent, confusing and jarring with no sense of direction, purpose, or logic. Every application, whether from a third party or from Apple itself, has its own way of doing things, and not two applications are alike. They use the same widgets in different places; or the same widgets in the same place, but they do different things.
Sometimes, a button performs a single action; at others times, a button that looks exactly alike will open a menu (cleverly rebranded as 'popovers'). Most of the time, a 'back'-button is shaped like an arrow - at other times, it's shaped like a normal button. Some Apple applications have their own whimsical themes (the diary application looks like a diary, the calendar looks like a calendar), which only makes things more confusing because widgets are themed as well. Often you can't tell just by looking at a widget what it will do.
And let's not even get started about third-party applications. Third-party iOS developers seem to think that their applications live on islands; that they are isolated from everything else. Every third-party application has its own learning curve, because they theme things differently, use different and custom widgets nobody else uses, put them in different places, and so on. Some use a 'Done' button to go back to the application's main screen, others use a back button. Some have their preferences in-application; some integrate their preferences into the main Settings application; some have them both in-application and in Settings; and even some others, the most insipid ones, have some preferences in-application, and some in Settings. It's always a surprise where the next application dumps them.
The end result is that using iOS on the iPad is jarring. You have to switch from one type of user interface to another every time you switch applications, and this switch is often jarring.
It would seem that without clear human interface guidelines (we'll get to those in a minute), developers basically do whatever they please, from utterly random and arbitrary widget placement, all the way down to the cardinal sin of developing their own widgets from scratch instead of using the ones provided by Apple. In my orthodox and, perhaps, outdated and get-off-my-lawn view of UI design, the 'parent' company (or organisation in case of open source software) does the UI testing, writes the HIG, and shows how it's done - third parties then follow suit. This worked wonders in the earlier days of Mac OS X - and it stopped working wonders as soon as Apple started ignoring its own Mac OS X HIG (I shan't ever forgive Apple for brushed metal and the wood panelling).
Why am I being so strict about this, while few other (actually major) reviewers pick up on this? I get the feeling that other reviewers - from the large sites, mostly - have a tendency to be wowed by the shiny factor, without actually knowing a whole lot about proper UI design. And, of course, their invites to Apple press events are based on how kind they are to the company. To use John Gruber's recent pet peeve - the iPad is being graded on a curve.
People who used to shake their fists at inconsistent and confusing UI design, people who stood next to me on the barricades to promote coherency and predictability, now smile nervously as they backpedal and slowly back away. I've written about this before, actually. After the arrival and popularity of iOS, consistency became a dirty word practically overnight. Pre-iOS, people blasted Windows and its application ecosystem for being inconsistent (and rightfully so). Post-iOS, and these same people never talk of it ever again. It's such a massive shift in user interface design that I'm surprised nobody ever really talks about it.
This also isn't something that only applies to the iPad or iOS - no, other mobile operating systems, whether on smartphones or on tablets, suffer from the same issues. People often like to say that Android is an inconsistent mess compared to iOS, but in my view, they both seem to sport a very unhealthy aversion against what was once considered proper user interface design.
I'm pretty sure the web has had its influence here. The iOS App Store, after all, mirrors the web: individual websites may look pretty neat, but in the end, much more inconsistent than the web you cannot get - and it bothers me on the web just as much as it bothers me here. The web is simply not a good example to follow, since most web developers have absolutely no clue about UI design. They may understand design, but that's not the same as understanding UI design. They're two completely different things.
The end result is that while there are definitely a number of great-looking iOS applications, there is no overarching vision tying it all together; it's a mish-mash of wildly different approaches to UI design, all firmly rooted in a paradigm (WIMP/desktop) that simply isn't suited for tablets. So, not only are they uncomfortable to use - each and every one of them is a different kind of uncomfortable from the other.
As much as I have my doubts about the strict approach Apple takes with its application store, they had the golden opportunity to design and write a comprehensive HIG for an interface designed specifically for tablets, with fresh and new ideas adapted to how people use tablets. Instead, they opted to shoehorn a traditional interface into a tablet, and then didn't even take the time to adhere to the staples of proper user interface design.
Curiously enough, then, that there actually are human interface guidelines for iOS, and they are a reflection of the inconsistent and jarring nature of iOS itself. For instance, the HIG advocates consistency at some points, while at other points promoting customisation and the use of non-standard controls.
As another example, the HIG also has a few words to say about where application preferences go (in-application or in Settings), but does so in a rather vague way, stating that preferences go in Settings when they are of the 'set once and rarely change'-type. Without more guidance, it's clear many application developers don't really know what goes where, and some just dump everything in one place, or in both places.
Another example which illustrates just how unimportant the HIG is to Apple is the use of contours around clickable buttons. The HIG states that "iOS controls, such as buttons, pickers, and sliders, have contours and gradients that invite touches". Yet, arguably the two most prominent iOS applications - Safari and Mail - both violate this rule. Clearly, Apple itself doesn't really adhere to its own HIG either.
The iOS review guidelines, which iOS App Store reviewers use to accept or reject applications, makes it possible for Apple to reject applications that do not follow the HIG. For instance, they state that "apps must comply with all terms and conditions explained in the Apple iPhone Human Interface Guidelines and the Apple iPad Human Interface Guidelines". However, the HIG is inconsistent in and of itself, making it rather impossible to actually violate the HIG in the first place.
I'm not dissecting the entire iOS human interface guidelines in this article; I'll save that for a possible later one. What I wanted to illustrate is that you can't expect third party iOS application developers to be consistent and use standard controls, when the HIG they are supposed to adhere to is inconsistent and schizophrenic, and Apple itself doesn't bother to adhere to it either.
You know - monkey see, monkey do.