1. easy to learn, fast to write: it should be as fast as writing HTML (the farther it goes from HTML the worse is)
2. programming logic should be not visible (no PHP or ASP code, no sql – is xsl/xpath simple enought?)
3. fully documented and supported by an active user community: if you have a problem, you should be able to solve it quickly, maybe with help from some community help or knowledge base like a wiki, a forum or a manual)
4. editable with preview with wysiwyg tools (like dreamweaver): you don’t have to change HTML programmer habits
5. errors should not block all the template rendering
6. errors should be easy to identify and correct
7. templating language you choose should be a standard or widely used (quite important for your selling)
8. there should be templates, functions, addon or macro (I mean something that can help you to make difficoult tasks easy)
9. fast rendering (maybe It should let you to control caching logics)
10. different outouts (html, xml, text and so on)

smarty, cheetah, freemarker, velocity, webmacro…