YouPorn does have competitors. Let's take a look (and see how many double entendres I inadvertently use)
RedTube - First totally free porn site I found.
Pro's - Dead simple UI. Blazing fast load times. You can scrub through videos quickly (to skip over boring parts, this is very important). Also, Looong videos.
Con's - Lots of repeat videos. Few ones that you don't see everywhere else. Not too many ways to drill down through the content. They sacrifice user control for simplicity.
Note: Arrington always mentions this one by name when writing about the porn industry.
Spankwire - Digg for porn. Hard to tell though, their UI is awful.
Pro's - Blazing fast load times. Loooong videos. Ability to scrub through them as they play quickly. Also, a really deep, varied selection of videos. User comments on videos are actually intelligent.
Con's - Doesn't update as frequently as other sites. UI is imprenetrable. I've never voted on anything. I wouldn't know what to click.
Xvideos - Big player.
Pro's - Adds about 30 videos each day. Crazy deep selection of videos. Decent tagging system. Related video algo is pretty good. UI is pretty ugly but serves its purpose.
Con's - Video's are slow to load. You can't skip through them without waiting for them to buffer all over again. Major performance issues here.
Megarotic - One of the big semi-pay sites.
Pro's - Incredible video selection. Really good performance and playback. Can scrub through the videos.
Con's - Costs money to watch more than 4 videos a day on their site. Their video player is getting more and more proprietary, adding popup ads, massive popup controls and branding.
Fantasti.cc - This site's incredible. Social network for porn and people looking for casual sex. Rather than power their own videos they simply scrap embed codes from Xvideos, Pornub and Megarotic and tack on a slick UI and social networking features. I think all this could be done with Drupal and a few modules.
Pro's - Well-designed. Really nicely designed. Enormous selection of videos by sucking in everything on other sites including xvideos, pornhub and youporn. There's also an actual user community of people making collections of videos (a great feature I've not seen anywhere else, letting humans organize and categorize them into weird subsections like "Chicks with flip - flops"). Users review videos. They also create user profiles where they post up amateur photos and video clips. The slick UI sets it all apart from other sites that do the same.
Cons - Because they steal from all the other free sites, videos disappear from the DB. Also, performance is only as good as the sites they steal from. I bet 60% of their content comes from XVideos...which means performance is often terrible.
Pornhub - Slick video site.
Pros - Well-organized. Fast playback. Good selection.
Cons - Has the same videos as everyone else. Front page features stuff that people just watched, so you end up seeing the same clips over and over on the front page.
Conclusion from my own observations: There are loads of competitors. The ability to scrub through videos is crucial but still not available on all sites. Offering a simple UI is important but without advanced search or other organizing features like Fantasti.cc's wonderful user collections one risks losing users (I rarely go to redtube. It's too basic and hard to find new stuff.)