I know there's a feedback button, but it strikes me that it might be useful to have a thread allowing for discussion rather than a bunch of independent feedback responses. I'm not really sure where to put it given that this is technically for "client help" and not suggestions, so please move it somewhere more appropriate if this isn't the right place.
Having fiddled with the HTML5 client now, there are a few places where I think some clear improvements could be made without too much work.
(1) Documentation. This is a no-brainer. Right now, this is probably the single greatest headache. It doesn't need a manual (though that would be nice), but there has to be a resource where I can just see all of the methods in the client object laid out in a simple way. It took me nearly half an hour of digging through the code by hand to find out that the way to retrieve the client's target was client.current_target(). That's unacceptable. This should be priority #1. Whoever is in charge of this should stop working on anything else and just get the documentation done - waiting until the client is "finished" is going to mean never having documentation ever.
(2) More documentation. Basic functions would be nice, but what would be wonderful is documentation allowing us to easily write our own widgets for those beautiful little frames. The ones that already exist are nice, but if it were relatively easy to write a limb counter display and get it to show up in one of those tabbed frames? Get out of town.
(3) Packages and groups. Reflex packages are groups, or at least they should be. Groups should be easy to package and export. The division between packages and groups isn't doing anything right now except adding needless complexity to the system and clunkiness to the UI (since you have to use that drop-down menu to switch between packages rather than the quite nice UI for opening and closing groups). Get rid of "packages" and make it possible to export any group. When you import a group, it should just appear as a top-level group like any other group.
(4) Add scripting to keybinds. This one is just mysterious - keybinds should have the same action types as aliases rather than only being able to send commands. I imagine this is just an oversight. Right now, having to make a function and call it via a command if I want a keybind to run code is very messy.
New functionality is always nice, but I honestly think the HTML5 client is already plenty powerful for very nearly anything you could want to do. The few features it doesn't offer pale in comparison to the problems of usability. What the HTML5 client needs right now is not more features, it needs documentation and it needs simplification. The package/group distinction isn't doing anything useful. The concept of "functions" is, if anything, doing active harm to the usability of the whole thing. onLoad is an incredibly clunky way to handle an absolutely essential function (inserting variable and function declarations into the global namespace on load).
These are the things holding the HTML5 client back, not lack of features.