The conversion should be loss less. This is a very challenging part of the process and an area where most of the converters fail. The next thing is speed. Now I want to expand on this. It is really easy to be have a high speed conversion if your the only one using the site. Where most of these conversion sites fail is when they have to process multiple conversion at once. Sometimes even several 1000 conversions at the same time. Most of the lower quality conversion sites have shared hosting. It is almost impossible to provide a solid Youtube to mp3 converter without a dedicated server. And in a lot of cases many of them. This is my point on the resources. What good is a conversion site if it locks up every time you want to use it. And this is the case with most of these sites. They have inadequate resources to provide the service they are advertising.
The final thing I want to touch on is the user experience. It should be fast and most of all easy to use. I have no idea why some of these companies lay their sites out the way they do. They use awful templates that don't lend themselves to the design of the site. And then they just throw up ads all over the site in a random fashion. None of it makes any sense. The site that gets 4 stars in all of these categories is Video2mp3.net. This website has been around for a long time. It is considered by most people as the leader in this field. These guys actually spend time developing and improving the process of file conversion technology. They use a ton of really cool ways to break the file down into little pieces in order to convert it faster. And then they re assemble it. In my experience Video2mp3 is the fastest Youtube to Mp3 converter on the market. They also care about customer service and stay very active on their facebook fan page. One of the things they offer is a subscription service. They offer a subscription that is really cheap and eliminates all the ads. So you can convert with zero advertising.
Since this service is a free service they have to find a way to pay for their service. And it is a premium service. They do not require a registration to use the site. You can simply visit the site and start your conversion mission. Overall they have the least amount of downtime and honestly if your looking for a youtube to mp3 converter then check out video2mp3.net. They really are the very best in this area and the leader in file conversion technology.