Not to be argumentative, but...
1. It runs slow as molasses in winter
Which part, on the server side or client side? Server side it seems to run like a JSP app, the first hit is slow while it compiles it. I set up the server on a very old machine with little memory, and had no issues at all after the first hit. I had no issues on the client side either.
2. It requires you to learn its own markup language, which encourages a mixture of style, structure and logic
It mixes them no more than HTML does. If you do it right, it can be done very cleanly.
3. I hate getting tied to a plugin when I don't need it
If you don't need it, then why would you use it? I wouldn't use Laszlo, or any other technology, without a good reason. To me Laszlo is to flash as JSP is to servlets. There is a time to use JSP, and a time to use servlets instead.
4. This seems to try to side-step all standards web developers fight for
Looking at my server logs, flash IS a standard. Because one company controls it I can be assured that my flash app will run cross platform and on different browsers. I would use Flash over DHTML any day because it is more reliable.
5. Now that we have AJAX-centric toolkits... do we really need something that depends on a closed-source plugin
Does it seriously matter? Flash is a compiler and interpreter, just like Java... and I don't see anyone complaining about the Java VM being closed. Ajax and flash are also very different animals, I can think of many situations where one of those would be helpful, but not the other.
The bottom line is that there is a right tool for every job, and maybe at some point Laszlo will be the tool I need. The key for me is that it is like XUL but is delivered as flash, which is currently what my users support.