Operating SystemTwo major OS are Google powered Android (opensource) and Apple powered IOS. In case of android running devices, check out its lastest version, v4.1 (jelly bean) is the lastest version of android.
ProcessorNew generation tablets have a processor of 1ghz or more, all new Google Nexus 7 have a quadcore processor of 1.3 ghz.
Screen SizeTablets comes in range of 6" to 12" , screen size really effects the price of a tablet. So while buying a $200, you should not expect a 10" screen.
Build and shapeWhile Ipad looks really good but its really brittle. Whereas Google Nexus 7 and Samsung's tablets proves to be relatively strong.
Price and Value for moneySome of tablets such as Ipad are priced more than it really worth. All other tablets are competitevely priced.
CameraAlthough you may not require this feature in a tablet, but it is required in case of video chat and many other apps.
Calling facilitySome of the tablets like Samsung Galaxy tablet have calling facility.
USB SlotMost of the tablets as Google Nexus 7 , Samsung Galaxy etc have USB slot but Ipad don't have a USB slot.
Availability of accessoriesWhile buying an tablet you should first consider that whether its accesories and parts are available easily or not.
BrandIts the most important thing, as a product from reputed brand is always durable.
A Technology blog