(Zepto is broken across multiple files so I might be missing some functionality.)
Roughly, Zepto has this many properties/method: On $: 5, On $.fn: 32.
jQuery has: On $: 95, On $.fn: 146.
Phrases like "jQuery-compatible syntax (lots of the jQuery API supported!)" make it seem like you can just rip jQuery out of an application and drop Zepto in - and it'll just work. Zepto only supports about 15% of the jQuery API - I'm very skeptical that any major jQuery plugin would "just work" given the provided API.
Framing the marketing within the context of jQuery (or, really, even Prototype) is disingenuous and creates a deceptive message. A better way to market the code would be: "Zepto provides a simple API for making JavaScript awesome in the latest WebKit mobile browsers - all in 2KB!"
That's 100% accurate and frames the code in a way that users will be able to grasp (2KB is nothing! It'll make my code simpler when I build my next PhoneGap app!). Whereas framing it within the context of jQuery doesn't benefit Zepto - it doesn't support more than one platform and it doesn't support the full jQuery API.
Zepto has been positioned as compatible with jQuery - inviting opinion on the subject.
As the comparison was one of the core selling points of the argument for Zepto, it's actually wide open for discussion.
That's true. Lot's of it is. Some of it is not. And yes, it targets bleeding edge WebKit mobile browsers.
Yes, some of it is broken and doesn't work correctly yet.
Some of it fixes bugs that jQuery doesn't fix AFAIK (like memory leaks on asset removal on iOS devices).
It's an early beta.
And it does it all in 2K.
I don't care about what the jQuery project does, and how it takes "a far more pragmatic view of web development"; Zepto.js doesn't want to be a do-it-all-works-everywhere-project; so I really don't get your condescending tone ("just taking a quick peak...", "straight-out breaks", "missing critical functionality", "broken for most cases", "I can keep on going", "...might be fine").