By Iain Mackenzie
Newsbeat US reporter
Some users are unhappy that older videos play with black bars on the side
It's a weird contradiction with the internet. The online world is supposed to be dynamic, constantly changing, open to new ideas.
Yet every time something moves forward, there seems to be an outcry. That's what's happened with YouTube.
The site has introduced widescreen, high-definition (HD) video playback.
For fans of numbers with an x in the middle, it means users can now view video in 1280 x 720 resolution.
Previously the maximum pixel count was 480 x 360.
However, some YouTube users have reacted angrily to the change and have been complaining on the site's blog page.
The black bars are distracting and annoying
Shane 1163 writes: "I hate how it looks. If you're gonna have that give us an option to get rid of it!!!!!!!!!"
Marcor5000 added: "The black bars are distracting and annoying and more annoying than the black bars when you use 'letterbox' to view 16:9 on a 4:3 television.
"You could also let people choose in which aspect ratio their videos should be uploaded in."
'Powerful viewing experience'
Of the 57 comments posted on YouTube's blog, most contain some element of criticism.
A YouTube statement, published on the site reads: "This new, wider player is in a widescreen aspect ratio which we hope will provide you with a cleaner, more powerful viewing experience.
"And don't worry, your 4:3 aspect ratio videos will play just fine in this new player."
Many users object to the addition of black 'pillarbox' bars at the side of older 4x3 aspect ratio videos.
Previously, widescreen clips have featured 'letterbox' bars at the top and bottom.
Some users have suggested the switchover is for commercial reasons.
Shane 1163 writes: "These guys are doing this so you can make it more professional so they can cut deals with producers from Hollywood."
YouTube has recently agreed a partnership deal with MGM, Lionsgate films and TV broadcaster CBS, which paves the way for full length HD movies and television shows being distributed through the site.
Upgrading YouTube to cater for higher resolutions is also seen as a move to combat the growing popularity of other online video sites, including Dailymotion and Vimeo.
Despite the criticism of its new aspect ratio, there has been praise for the upgrade to high-definition.
Nataliana1234 wrote: "Awesome love upgrade can't wait for HD currently use Vimeo for that."
YouTube has made major changes to its technical specifications in the past.
Since March 2008, users have been able to access some videos in a "high quality" format - more than twice the resolution of its original video offering.
Around the same time, in addition to the flash video format used since launch, YouTube began encoding videos using Apple's preferred h.264 system.
This allowed clips to be viewed on the iPhone, iPod Touch and Apple TV.
As well as providing better video resolution, h.264 also allows higher quality, stereo audio.