OK , i will be a little more specific and here it goes..
Powerful - I do not intend to use the arduino board to intense tasks. Neither i want to use it for just switching a light ON/OFF. But somewhere in the middle. I intend to use the board to run a couple of tasks/application that enumerate multi-tasking. I want the board to multi-task and not just run a single infinte loop. By doing so , I want the board to be connencted in a network and control various other appliances. Essentially I am looking for something more powerful than the UNO and less powerful than the PI. Somewhere in the middle.
Memory - Since I want to be doing multi-tasking, memory is another factor that i need to look into. The onboard memory should be capable of running the program without any lag. An external memory might also come handy( a micro sd card interface).
OS - An OS if present will take care of all running pheriperals in the device. Hence i thought an OS will be wonderful.
Communication - Also since I am going to be using the arduino to communicate with other device, communication should not be a hassle. I dont want to buy shields or other add-ons for communication. It should be a good board for communicating right out of the box.
The above all features are present in the YUN. But the only problem is that the YUN has a not so powerful processor !!