First post here. Kept getting ominous-sounding emails from FM TechNet reminding me that "You haven't created any content yet." So, here goes…
Having had a chance to play around with pre-release version and now, finally free to discuss the new features, I thought I'd share a few observations about the new layout design environment. The CliffsNotes version is that the new CSS design capabilities are generally powerful, not to mention fun to play with, but come with a learning curve and some unexpected gotchas.
First, grids and guides. The new static, shared, and dynamic guides are extremely welcome and should be instantly familiar if you've ever used, say, Illustrator. Grids, otoh, now behave rather unexpectedly. If, for example, you set a grid spacing to simulate the old 6px interval (say, major interval of 72 pt with 12 subdivisions, or just 6 and 1) and begin dragging an object while watching the new coordinates tooltip, you'll notice that the object may snap in unexpected increments, such as 3 px, or 2 and 4 px alternating. I believe what's happening is that the drawing engine is alternately snapping the top and bottom object edges (or left and right) to the grid, such that, unless the object's dimensions are an exact multiple of the grid interval, this is what you'll see. If grids are your traditional comfort zone, this will certainly take some getting used to. My preliminary recommendation is to turn off grids and embrace guides in their various flavors. (And it goes without saying that you should not turn on both snap-to-grid and dynamic guides at the same time, lest you immediately find yourself in "snap-to" hell.)
Next, themes. Again, these are powerful and welcome, but with caveats. If you're persnickety about your layout aesthetics and think you just might ignore the new themes altogether, in order to roll your own personal "look and feel," think again. Themes are not optional — every layout starts with an underlying theme and that theme controls many aspects of a layout's appearance that are not directly under your control. (But do see Richard Carlton's thread in this forum re the possibility of editing the underlying css files and adding customs themes — something about which there will probably be a lot more discussion in the coming weeks and months.) For example, effects such as engraving and embossing are no longer under your control in the inspector, but these effects are not actually gone. They are embedded into themes, with each theme being optimized for the proper amount of such effects. Other attributes that are embedded and not exposed for direct editing include the look of tabs in tab controls, the look of the drop-down and calendar widget buttons in combo boxes, the color of the "in-focus" glow that appears around active fields in Browse mode, and the pin-stripe background pattern of certain themes. Thus, while you can edit many attributes of layout objects, such as the new (and extremely welcome!) gradients and dynamic corner rounding, many attributes simply come with whatever theme underlies your current layout.
Another example of layout attributes out of your control is important enough to warrant its own paragraph, that being text padding within field controls. Each theme has a default font, which, of course, is under your control. Each theme also has automatic padding within edit boxes and other controls, which is not under your control and is optimized for the theme's default font. The result is that if you decide that you like, say, the "Onyx" theme (which I do) but want to change the font from Helvetica to Verdana (which I did), the padding remains optimized for the former, resulting in the text sitting lower in the field control. The field will initially automatically expand 3 pixels (um, points) to accommodate this, but if you change its height back to the original dimensions, the text now sits unattractively low in the control. Short of editing the underlying css, it's probably best to stick with the font for which your chosen theme is optimized.
Lastly, what happens when you copy objects between layouts with different themes applied? In some cases (field controls, for example), the object retains the look of its originating layout's theme. (This could be a handy way for getting, say, field controls with a different font spacing or different amount of engraving onto a layout.) In other cases (tab controls, for example), the object adopts the look of the destination layout's theme. In either case, if the object had customizations applied before copying, all bets are off, trial and error being necessary (at least in my limited testing).
Mind you, none of this is to say that the beautiful, new themes and their underlying css technology aren't big steps forward, just to warn that there will be a learning curve until we, individually and collectively, have wrapped our heads around the new ways.