Eric begins by asking about the names. How do they always come up with funny names like Gatekeeper, JSMeter, and so on? Ben Zorn explains that it is important to pick a good name, because they tend to stick in people's minds better than paper titles (unless authored by Phil Wadler). I agree. That said, I would much rather have a boring paper title than a ridiculous backronym project name, the likes of which are way too common on large projects in the sciences.
I'm not objectively sure what "short" is, but we did see in our work that function size was fairly consistent with respect to static code size and dynamic bytecode stream length. We could not distinguish function invocations which were callbacks though, unfortunately. I also wonder if opportunities for method-level JIT'ing are overrepresented in SunSpider and (especially) V8 benchmarks.
I agree that it is bounded by other technologies, but it can also be a general-purpose scripting language (see for instance its use in writing large parts of Firefox, and as a scripting plugin for many environments and games). I think the issue of poorly designed semantics (the root of all trickiness in efficient implementation) is orthogonal to the issue of whether it's a generally expressive and useful language. PHP is another language in this vein (apparently useful, but horrible semantics).
One lesson of JSMeter that Ben Livshits talks about is the possible benefits of more integration between the language and the platform. Many times, browsers load the same page over and over, but do not learn anything about how that page actually behaved. Ben's example is that if code only runs for a few seconds, then it is not useful to run the garbage collector (as opposed to other methods, such as mass freeing by killing a process or using arena/slab allocation). Right now, browsers are utterly amnesic about what happened the last time (or 10, or 1000 times) they loaded a page, and only cache the source text on the browser (as opposed to the parsed AST). This is something that jumped out at me as well. Sounds like an interesting thing to look at. They talk about this again near the end.