In informal writing I capitalize only two things: the beginnings of sentences and the word 'I'. This style is called "minimal capitalization" [1] and has several advantages. The primary advantage is that it is a good tradeoff between minimizing use of the 'shift' key while still allowing sentence breaks to be clear. Other advantages include not needing to take sides in various arguments that use capitalization as a battleground, simplifying the process of writing by not needing to consider whether a noun is proper, and being the appropriate next step in a tradition of capitalization reducing orthographic reforms dating back to the 18th century.

Update 2012-01-06: I've mostly stopped doing this. I have some budget for nonconformity in readers minds, and why should I waste it on capitalization?