In my view mobile devices in general have regressed in a lot of major ways even though I still enjoy using them.

• Battery Life - There has been some improvement in this area as of recently but in general battery life is pretty terrible on modern smartphones.
• Cameras - Most smartphones have pretty basic cameras. Most smartphones only have a single led flash, some don't even have a flash. Dual led flashes are little bit better but nothing compares to a true xenon flash when it comes to low light performance. It can also be nice to have both led and xenon flashes and to have the ability to switch between either a xenon or led flash. The size of camera lenses haven't really gotten bigger and have in fact gotten smaller while megapixels have increased. Camera lense covers are even rarer than the other mentioned features. Shutter buttons are a real time saver and most phones simply don't have them unfortunately. True optical zoom is also very rare on smartphones, optical zoom preserves quality as one zooms. Nokia pureview devices while better than most devices lack optical zoom and lense covers. In my view the best camera phone was the Sony Ericsson Satio which took great pictures and had all the features I mentioned, maybe one day Sony will release an Android based successor. Some Android devices from Samsung and polaroid check all the necessary boxes like the Galaxy Camera but have limited availability. Tablets almost universally have bad cameras with no flashes and the lowest possible quality, a camera on a tablet can be useful when no other devices are with you or for taking notes during class. I do realize that not everyone desires those features but many people do desire those features. Even though imaging has gotten worse video recording is much better than used to be in terms of both quality and resolution. Having a good camera on either a tablet or a phone is useful so you don't carry an extra device and dedicated are easily forgetable.
• Os performance - Modern Mobile oses are real hogs when it comes to performance and usage of computing resources. Whenever I use an Android device, almost half of the ram is used by the os and the Android os takes up about as much space as a install of desktop linux. You can still get bad performance even on the most modern of devices. Older oses ran very well on very limited hardware and reaped the full benefits of performance increases, Windows mobile 6.5 (not phone!) ran really well of 1st generation snapdragons as well as older devices and Symbian ran really well on the omap 3 platform as well ad older devices. Ubuntu touch runs well in real world use but has very high system requirements. Windows 8 runs well on smaller tablets but in practice uses almost all the built in storage. BlackBerry 10 is overall the best in terms of both real world performance and optimization of performance in my experience. Even though modern oses are more resource hungry they aren't much more functional than older oses.
• Desktop Syncing - This feature is terribly implemented by all mobile oses and it is one of the most ultra basic. With every Android device I have owned I have errors simply transferring files becomes a choor because of issues detecting my device, I have had problems of multiple versions of Android and on Windows, linux, and mac. On macs Android devices simply won't show up in many cases. On linux Android devices show up when connected but can have errors like it suddenly not working. With iPhones itunes is generally a terrible program with alternatives available. BlackBerry 10 devices are the worst and will allow file transfers on linux if you connect a bb10 device as a SMB device. Windows Mobile devices and Palm os devoces implemented this feature perfectly, they each had their own desktop suites with available alternatives thst could manage everything on your device such as contacts, emails and what have you. Palm os and Windows Mobile generally didn't have errors with file transfers. I don't get why such a basic feature is so poorly implemented.
• SD Card Support - Most devices come with a sd card slot but software wise the feature is poorly implemented. The apps 2 sd feature has been enabled and disabled many times by google with various versions of Android with overall software level support in general being basic. Some apps on Android can be put on the SD card, but many can't and in some cases apps can't even read data from the SD card. Newer versions of Android habe partially addressed this but it remains a problem. SD cards are especially useful with large games and media files. The implementation is similar with Windows Phone 8 and BlackBerry 10 which is to say inconsistent and limited. Windows Mobile 6.5 or older implemented the feature very well, in general all apps could be installed on either the internal or external storage and one could choose during installation. This is not really a hardware but a software problem.
• TV Out - There is very little standardization here and there are often scaling issues due to significant differences between the resolution and aspect ratios of tvs and mobile devices. There are many competing standards such as mhl, Samsungs own standard, micro hdmi, mini hdmi, proprietary port to hdmi and in some rare cases full hdmi.
• Stylus support - On a software level there are many great applications for both Windows and Android that take advantage of a stylus or digitizer but the number of devices available with it are limited, I always make a tablet buying decision based on whether a stylus. Hopefully more devices in the future have it.