When the very first version of this popular operating system was launched, it was obviously not matching contemporary operating systems being used in other popular mobiles including Windows, Symbian, BlackBerry and Palm OS.
It didn't support multi-tasking, third party applications and even 3G.
Some very basic functions of Smartphones like copy and paste text were also not there.
Even adding files in the mails was quite inconvenient.
Some other things that had become very common by then but were not supported by this version of the operating system are:-

• Exchange push mail
• MMS, voice dialing
• Personalization of home screen

The main things focused by Apple were user interface and speed.
Three innovations by Apple gained utmost popularity for it and impacted the mobile industry significantly; they were the magnificent user interface, Safari Web Browser for mobile and widescreen iPod.
SO the main features were:-

• Good user interface
• Speed

iOS 2 This one was released in July, 2008, which became yet another historical moment for Apple.
The third party applications had become a very common thing for Smartphones by then but Apple took it to the next level by allowing the users of iPhones to develop browse and install apps in their own application store.
iOS 3 In its endeavor to provide better user experience, Apple introduced iOS 3 in June, 2009.
This time the company took the great demand of 3G users in mind and created the latest version of its operating system with 3G support.
Later with a series of upgrades, it was blessed with a few more features like:-

• Spotlight search
• MS support
• Pushing notifications of third party apps
• Voice playback]

iOS 4 It was unveiled in June, 2010, with a view to offer something that meets the expectations of advanced users.
The features newly included in the version were:-

• Multitasking
• Wi-Fi bonding
• Application folders
• Spell checker
• Unified inbox
• Customizable search
• Multiple exchange account
• Retina display
• FaceTime

Though multitasking was the biggest highlight of the version but not allowing applications running in the background was a big drawback.
iOS 5 Here comes the game changer! This version helped Apple in capturing a great share of market in an immensely competitive environment.
It was filled with plenty of new features including:-

• Siri
• iMessage
• iTunes
• iCloud
• Notification center
• Wi-Fi sync
• OTA upgrade
• The advent of this version shook the mobile market and gave rise to a new type of competition.
  

iOS6 The most talked about thing in this version was the inclusion of Apple Map.
Apple developed its own map service due to the lawsuit with Google.
But the company couldn't get it right in the first go and the app was not liked by users in the beginning.
Things went so intense that the President Cook himself had to issue an apology statement.
iOS 7 iOS 7 changed the look and feel of the system absolutely.
Besides brighter colors and enhanced UI, a couple of other things were also changed.
Apple tried to make this one to be capable enough to penetrate the highly competitive mobile market by including several innovative features like:-

• iTunes Radio
• Touch ID
• AirDrop

And as all other versions, it was also taken up keenly by the customers.