This is what mockup software is for i.e. Balsamiq. Once you have what you want there, then you go to Photoshop. Then export your pretty images, insert them into Balsamiq and keep tweaking.
This is why Photoshop and Illustrator are still the best tools for web design: they enable relatively quick wireframing, but also give you the possibility of increasing the level of polish within the same document.
If a new alternative is to appear, it will need to provide at least support for realistic grids, typography, and basic CSS styles, not just empty boxes and Comic Sans.
Dynamic layout - as far as I can tell balsamiq uses an entirely static layout, unless I'm missing something.
Viewport preview - again can't see how it helps with this
Text dimensions - have to be specified in pixels, not ems.
and these are just examples of some features that are missing...