The following 5 things would really help developers to create better web applications.
1. Standard Libraries
We should take the core of Dojo or something similar and embed it in all browsers so it doesn't have to be downloaded on every web page. It should not be as big as full Dojo, and it should be stable.
2. Server Push
Comet is too hard. Reverse Ajax in DWR is one of the trickiest bits of code I've ever written. It should not be so hard.
3. Better Widgets
Menus, trees, sortable tables, tabs, draggable divs. The What-WG is working on some of them, but by no means all.
4. Vector Graphics
Microsoft? Have you got beyond "we're thinking about it?". Opera, Firefox and Safari/KHTML all have Canvas and SVG either in a full release or in CVS.
5. Local Storage
What did I miss?