When the X axis labels are text strings, it happens often that they cluster together and they become very difficult to read.
For this reason, I usually have to use an option like + opts(axis.text.x=theme_text(angle=60, hjust=1.2, )) in order to draw them horizontally and make them more readable.
Consider making this the default behaviour for when the X axis labels are long strings. One of the strongest points of ggplot2 is that it has very nice default options, so this will improve the readibility of a ggplot2 figure.
It should be possible for a nicely design axis guide to this automatically, particularly if it could use Justin Talbot's algorithms. I've assigned to Kohske, but marked it as a long term to do.
That sounds like a great feature, but unfortunately we don't currently have the development bandwidth to support it. If you'd like to submit a pull request that implements this feature, please follow the instructions in the development vignette.