We looked into this a lot and found the same. AppleTVs are not that great for digital signage unless you want to develop for it. We use BrightSign (and BrightSignNetworks) for ours and they're great, even though the advanced formatting features require a Windows client (There is a basic web platform that works just fine for us presently). They're cheaper than the AppleTVs as well, though most folks ask this question because they already have AppleTVs. I'm also waiting on a beta for Dashman... which looks great but doesn't exist yet.
Seriously though, check out BrightSign.