On their page "continuing to improve our ads", the AdSense team explains the reasoning behind the changes. VP at eCreativa noted "While your reasons are understandable, the surprise factor is not...I recall how I made my technical manager check everything (time wasted for nothing) when ads started to appear with a border while the code was clearly not designed to do that."
The AdSense team reports on their blog the nature of the test and new features: "More specifically, changes such as redesigned ad units and arrows to show additional ads have stemmed from these tests."
I don't know who is going to press a down arrow to see more ads. I think Facebook'senhancement of its ads is much better: each ads allows you to vote for it (like it/don't like it) to fine tune your preference for targeted advertising. This is one step closer to the holy grail of advertising: targeting of ads only to the audience interested in them.