I would like to propose making tick marks point outward by default (and for the 2.0 release). The idea for this comes from William Cleveland's Elements of Graphing Data. I quote directly from him:

...tick marks should point outward. This format prevents data from being obscured (page 33)

The idea is really simple. Data belongs on the inside of the axes frame. If your tick marks point inwards, there is a good chance that the tick marks and data will interfere. This type of interference happens often in practice with dense amounts of data in line/scatter plots and with all kinds of color/surface plots. Here is a simple example:

The tick marks on a a default styled pcolor plot are almost impossible to see along some borders:

The tick marks outwards become extremely helpful and visible:

Cleveland's way of describing this choice also helps me to understand the default visual styling of ggplot2 and seaborn, which use a) no tick marks and b) light grid lines against a grey bg. This is simply a different way of providing the visual guides for plots that don't interfere with the data itself. The other option for the defaults would be to move in this direction (no tick marks with light grid lines on a grey bg), but I think that is probably too much change for the defaults.