'Never' seems like quite a generalization. What about e.g. Classify and Predict, or FormFunction, or Interpreter, or Nearest? Do those all not do quite what you need? Conversely, does other software do better, on average, in 'doing what one wants'? Which software?
There are gaps, definitely, one small example you've pointed out. My pet peeve, for example, is that it is unnecessarily difficult to map colors and sizes onto graph vertices based on vertex properties, which I needed to do for [1].
Still, the scope of visualization functionality is so vast [4] that it's not surprising there are gaps (which we continue to plug, the whole language is a never ending project). And having users report to us what they want lets us prioritize those things. Anyway, clearly DateListPlot should learn about error bars, and I'm sure in a year or two it will. Although it is frustrating you can't just go in there and tweak the source code yourself, I agree.
> Don't get me started on the KelvinsDifference vs. Kelvins thing, which are both represented by the same symbol to maximize your confusion when your unit conversions are failing...
I think Quantities should probably gain a little box, like DateObjects have [2], that lets you mouse-over for more information, which should make this problem easier to diagnose. But the distinction is apparently important [3], although I don't want to "get you started" :-). Either way, Alpha shouldn't change its interpretation from under you, that's definitely a bug.
> http://mathematica.stackexchange.com/questions/65600
Unfortunate, but fixed within a day of being reported! 10.0.2 has passed many weeks of SQA and will ship shortly. So at least this bug will only have been around for 0.0.2 versions :-)
[1] http://blog.stephenwolfram.com/2013/04/data-science-of-the-f...
[2] http://reference.wolfram.com/language/ref/DateObject.html, http://reference.wolfram.com/language/ref/Entity.html
[3] http://reference.wolfram.com/language/tutorial/TemperatureUn...
[4] http://reference.wolfram.com/language/guide/DataVisualizatio..., http://reference.wolfram.com/language/guide/FunctionVisualiz..., http://reference.wolfram.com/language/guide/ChartingAndInfor..., http://reference.wolfram.com/language/guide/MapsAndCartograp..., http://reference.wolfram.com/language/guide/DynamicVisualiza..., http://reference.wolfram.com/language/guide/GraphicsOptionsA..., http://reference.wolfram.com/language/guide/GraphicsInteract..., http://reference.wolfram.com/language/guide/SymbolicGraphics..., http://reference.wolfram.com/language/guide/ImportingAndExpo...