I am primarily interested in Mathematica as a development and communication medium. Someday most technical papers will be published in the form of CDF files or Mathematica notebooks, and a great deal of collaboration will be done through the medium of Mathematica Applications.
However, for this to come about more attention has to be paid to robustness and stability. It should be possible to get good enough at Mathematica that one would spend most of one's time thinking about the technical problem and much less time thinking about Mathematica. If the Mathematica overhead becomes too large then it will defeat the objective above and Mathematica will be used more as a calculator scratch pad with results copied out to other (inferior) media.
There are some nice features in Version 10 but also a few items that seem to be either retrogression or lack of progress.
Side Windows
We can produce very sophisticated dynamic presentations with Mathematica but these will generally require extended discussion in a notebook. If the diagrams are placed within the notebook then it is likely that they will scroll out of view when reading the discussion. So put them in a window beside the notebook. But this ability has been somewhat degraded in Version 10 in that side windows with dense graphics will be difficult to position with the mouse. They will not drag smoothly. Here is an example (made dynamically slow with PlotPoints):
CreateDocument[
Manipulate[
RegionPlot[Abs[x] < a Abs[y], {x, -2, 2}, {y, -2, 2},
PlotPoints -> 150, MaxRecursion -> 2], {a, 1, 4}],
WindowTitle -> "Jerky Positioning",
WindowSize -> All];
This will (at least in Windows 7) initially reposition smoothly but then eventually become very sticky and this seems a bit random. In Version 9 it can always be smoothly and quickly repositioned. It's as if the slowness of the dynamics has been transferred to the window as a whole. A retrogression.
Also, how does one control whether a menu toolbar appears?
SaveAs
In early versions of Mathematica, using SaveAs on an existing notebook always brought up the directory that contained the notebook. It is most often the case that the notebook will be saved in the same directory with the same or a different name. For example, style sheets can only be saved with SaveAs even though we usually want the same name.
Then in later versions of Mathematica this got broken and the SaveAs window went to the last folder that was browsed to. People complained and it got fixed. Now in Version 10 it has gotten broken again. Another retrogression.
WordCompletion
It used to be that when typing a Text cell one could use Ctrl+K to complete a word or more commonly to check its spelling. This got partly broken in Version 9 in that it worked but always generated an error message. A retrogression. In Version 10 it is completely broken - doesn't work at all. A further retrogression. If one is accustomed to adding discussion to their calculations (like writing a technical paper!) this is the loss of a valuable aid.
It also would be nice if one could spellcheck a selected part of a notebook rather than always having to go through the entire notebook, and always having to skip a number of items.
Workbench
Finally it's a bit disconcerting that Workbench has not been updated. I use it mainly for documentation. For serious projects - and I suppose most Mathematica users are working on serious projects - the Application format with documentation is a very good way to accumulate work in an organized manner. Workbench Version 2 is a bit ragged. For example it allows for multiple packages but accommodates them by an undocumented trick, which produces error messages that can be ignored. This is not going to be reassuring to most people. Apparently there is a Version 3 out there (and Mathematica documentation looks a little different than Application documentation) but even though I have a professional license, have always bought Premier service, have written a number of somewhat successful Applications, and am trying to help now with a major project, I don't seem to have any access to it.