Hope my thread is not too late for you to make your purchase.
AVI file has many different types. Just because it says it's an avi on the PC doesn't mean they are the same. Example? You can play some avi files on a PS3, but not others. AVI is what media geeks call a wrapper format, meaning it doesn't actually specify how you encode the video and audio into digital forms. If your TV/media player doesn't have the necessarily codecs for the particular avi file, it won't play it. Blurbs on the brochures may not necessarily go into details about which codecs the TV has, so very often you just wouldn't know what specific files the TV can play.
If you are trying to directly play a file on a TV, you should STRONGLY consider using a media player. It just saves a lot of hassles when you find out that the thing won't play when you plug the USB in. My personal guess is that in the market today many TV's have a USB port on, but the USB functionality doesn't offer much. It's usually just for viewing jpg files. Why don't the electronic makers produce one that does it all you say? Because if you want a device that plays all formats ultimately you would need to put a microprocessor chip in the TV that has decent computing power. That means cost, and it's hard to advertise to the popular audience. I've heard Toshiba has started to pull the CELL microprocessor chip in their TVs. That's the same chip that is in the PS3. It's only when you see things like this on a TV that you can feel confident that it does offer some sorts of multimedia functions.
So all in all, consider a media player. Read the box of the media player to see what kind of codecs it has, even if you have absolutely no idea what those are. Then kind of compare this specs on two or three other ones, and you would start to have an idea about which one does the job.