Do not forget that there are still lots of ways to get all the parallel I/O pins you want on a tablet... run a USB link to an Arduino.
No sense much trying to do a lot of numerical heavy lifting with an Arduino. It simply does not have the horsepower or memory for it. It can act as an intermediary between a tablet which has all sorts of horsepower, and a platform controlling motors and reading sensors.
If the application is quite menial ( say datalogging ), an Arduino can handle it quite nicely on its own when coupled with appropriate storage blocks - but in and of itself, just maintaining a FAT filesystem alone would be difficult for an Arduino, yet a piece of cake for a Raspberry Pi.
I am presently building with an Arduino platform and note I am taking a significant amount of its resources just to deal with two rotary quadrature encoders and two LCD displays.
I am aiming for absolute simplicity. I need lots of low speed I/O and bit-banging special protocols more than anything else ( and I can get it via Arduino's I2C bus ). I will continue with this, but if there is any significant numerical analysis or display, its going to have to partner with something else to do the heavy stuff.
As it is, I intend to use a Parallax Propeller chip if I exceed Arduino's capacity, as most of my needs are menial bit-banging protocols to interface old technologies to newer stuff - and I want it all done in parallel so I do not have interrupt, timing and latency issues. The Propeller chip has eight cores, running in parallel, so each core can be tasked with an individual menial thingie ( UART, SPI, I2C, video, audio, DMX lighting, whatever ), and they will run in parallel without contention or timing issues from waiting for the program counter to be handed to them.
Andre LaMothe has developed a "Chameleon" board combining an Arduino with a Propeller chip if you want to explore this avenue.
A Raspberry Pi would do everything. But then, sometimes a hand calculator comes in handy when you don't want to launch a fullbore compiler to evaluate some mathematical thingie you dream up.
I see a Raspberry Pi ideal for those places you would normally put a full-fledged tablet in... say an interactive kiosk with full display and TCP/IP networking. It has the horsepower to do darned near anything. And lots of hardware I/O as a bonus, where the Arduino solution involves channeling everything from the tablet through the USB bus or network link ( YellowJacket, DiamondBack, or similarly equipped Arduino ).
I guess one of the things I would like to see most is some sort of interface which would adapt to any LCD display out there and let me drive it with the Raspberry Pi output, as there always seems to be some defunct LCD display somewhere that I could repurpose.
Maybe something down the lines of this