On my company provided phone, all application icons look the same, the are heavy rounded squares. On my personal phone it is a bit better, only some of them are, the rest are circles. Despite not having very many applications (not even a handful in addition to the preinstalled once that couldn't be removed or was too fundamental to remove), it takes much longer for me to find the correct application on the phone comparied to the computer.
Icons have the following basic feature: shape, colour, texture. And they can of course have shapes on top of shapes. When all icons have the same enclosing shape, such as a circle or a rounded square (or hypercircle as the case may be), they look very similar because they the same primary shape. Typically, secondary shapes or not even outline, so they are not very prominent, and texture isn't a commonly use feature at all, in fact, it is difficult to include in small icons. This leaves colour as the primary distinguishing feature, and there are not that many colours to use. Yes, you can mix colours, but the icon will just look like the average of the colours. For this reason, icons for Google applications look almost identical.
When designing a the icon theme for a system, it is of course important to realise that how this feature's importance in distinguishing icons is different from person to person, and texture generally not useful at all for men. Conversely, having animated icons would problematic for men, as men are more sensitive to peripheral motion.
The importance of text associated with icons cannot be overstated. While this also varies from person to person, text may or may not be easier to find than icons. This may sound insignificant and may lead you to conclude that using only icons may be worth while to save space, however, find a piece of text in a list (or even on a page) may be take just a fraction of the time. Personally, I identify a text in a list about 20 times faster than an icon. Although I do realise that I've yet to meet someone that can find a piece of text as quick as I can.