It's been just over a year since Apple launched its Lion operating system, a revolutionary new operating system for Macs that was designed to mimic the popular, functional iOS software used on the iPhone and iPad. On Thursday, the company announced its first major upgrade to the Lion software, the OS X 10.8 Mountain Lion, which will introduce even more iOS features. The upgrade won't be released until later this year, but a preliminary version was made available today to developers and tech writers, who say the advancements will make the Mac "even more iPad-like." Here, five of the most exciting new features and changes:

1. Notification Center
The most obvious sign of iOS's influence on Mountain Lion is the Notification Center, which Harry McCracken at TIME calls "a consistent, centralized mechanism for calling your attention to things going on in applications other than the one you're using." The notifications briefly pop up in the top right hand corner of the screen to alert you to a new email or meeting scheduled on your calendar, for example, and an option lets you call up a scrolling list of all your recent notifications. The feature is "particularly useful when you're working in a program's full-screen mode and don't want to leave it in order to keep tabs on other apps."

2. iMessages replaces iChat
Mountain Lion will replace iChat with the popular iPhone and iPad instant messaging feature iMessage, touted as "Apple's version of BlackBerry Messenger" and previously incompatible with Macs. Like iChat, iMessages will let users sync with their AIM, Yahoo, and GoogleTalk networks, but will also let them send messages, videos, and photos to a contact's iPhone or iPad. "You or your friend can even begin a conversation via Messages on one device, then switch to a different one in mid-discussion," says McCracken.

3. Gatekeeper
Mountain Lion will also improve Macs' security defenses, says Chloe Albanesius at PC Mag. A feature called Gatekeeper allows users to "choose [their] level of security in an effort to avoid malware." The most-secure level will allow users to download only Mac App Store apps, while the least secure option will allow applications from anywhere.

4. AirPlay mirroring
A bonus for Mac owners who also own Apple TV boxes: AirPlay mirroring. This feature, already available on both the iPhone 4S and the iPad 2, wirelessly displays the contents of a Mac's screen on the screen of any HDTV set up with the Apple TV box. In other words, says Jason Snell at Computer World, "an Apple TV will soon also be a wireless display adapter for the Mac, letting you display webpages, YouTube videos, iTunes rentals, Keynote presentations, or anything else you can think of onto an HDTV without any added wires."

5. Documents in the iCloud
One of the most useful Mountain Lion features is one "that you might guess Macs would already have," says McCracken. Documents in the Cloud allows you to create, edit, view, and share documents and files from your iPhone, iPad, and your Mac. "Any changes made on any device will be reflected on all devices through iCloud," says Kent German at CNET. With Mountain Lion, you can also "create and access reminders and notes, and receive notifications" on all Apple devices, says Rich Jaroslovsky at Business Week.