The release of Big Sur was the first time the major version number of the operating system had been incremented since the Mac OS X Public Beta in 2000.

For the first time since OS X Yosemite six years earlier, macOS Big Sur features a user interface redesign.

To maintain backwards compatibility, macOS Big Sur identified itself as 10.16 to legacy software and in the browser user agent.

[13] Using these methods, it is possible to install macOS Big Sur on computers as old as a 2008 MacBook Pro and iMac and 2009 Mac Mini running smooth with non-metal graphics.

[14] macOS Big Sur refreshes the design of the user interface, described by Apple as the biggest change since the introduction of Mac OS X.

[18] An interface with quick toggles for Wi-Fi, Bluetooth, screen brightness and system volume has been added to the menu bar.

[23] There is no option to convert existing, HFS+-based backups to APFS; instead, users who want to benefit from the advantages of the new, APFS-based implementation of Time Machine need to start with a fresh volume.

[26] In the new version of Time Machine, encryption appears to be required (instead of merely optional) for local disks, but it remains elective for networked volumes.

[27] Non-core applications and user data can be restored in full using Migration Assistant, preceded by a system reinstall if necessary.

[28] Spotlight, the file system indexing-and-search mechanism introduced in Mac OS X 10.4 Tiger, is faster and the interface has been refined.

[31] This includes adding an SHA-256 hash for every file on the system volume, preventing changes from third-party entities and the end user.

[34] Safari 14 also includes built-in web page translations in English, Spanish, German, French, Russian, Chinese and Portuguese as well as support for 4K HDR content from Netflix on Macs with an Apple T2 chip, although none of these were made available for macOS Catalina and Mojave.

Safari 14 also supports WebExtensions API, the WebP image format as well as VP9 decoding, the latter of which allows for the playback of 4K and HDR content from YouTube.

[36] The Messages app was rewritten to be based upon Apple's Catalyst technology[37] to enable it to have feature parity with its iOS counterpart.

The new version of the app included a refined design as well as the ability to pin up to nine conversations that can sync across iOS, iPadOS and macOS.

[43][44][45] The initial rollout also disrupted Apple's app notarization process, causing slowdowns even on devices not running Big Sur.

[50] Certain Apple applications running on early versions of Big Sur were reported to bypass firewalls, raising privacy and security concerns.

[53][54] Conversely, security experts have reported that Big Sur will check an application's certificate every time it is run, degrading system performance.

[55] Apple responded that the process is part of efforts to protect users from malware embedded in applications downloaded outside of the Mac App Store.