I've been seeing a lot of random errors in capitalization lately -- these things do tend to come in waves, though it's anyone's guess as to why. I deal with writers all over the world through an assortment of classroom, workshop, private coaching, and other settings. These folks sure aren't being exposed to the same ideas or instruction that would lead to these errors suddenly cropping up. Just something in the air, I guess.
Lately, it's been overuse of caps, rather than under-use. Random common nouns, in particular, are getting heavyweight status. It would be as though in this Sentence we chose to cap -- well, you see it. This might be a result of chatspeak trends where people use caps to emphasize words, but it's probably not something you want creeping into your fiction unless you're writing something highly stylized and tightly controlled.
Use a capital letter:
* At the beginning of a new sentence
* For names and other proper nouns (Mary Smith, France)
* For formal titles when used in conjunction with names (Lady Mary Smith, Chief Wiggam)
* For the titles of specific geographic locations (Lake Michigan, Ohio Street) and for designations related to these locations (Italian-American, New Yorker)
* For the proper names of institutions, organizations, businesses, and government bodies (English Department at the University of Chicago, League of Women Voters, Doolittle’s Bar and Grill, Internal Revenue Service)
* At the beginning of a direct quotation (Mary said, “Let’s go to Doolittle’s.”)
* For the pronoun I
* For calendar items (Tuesday, Christmas Eve)
* For words in the title of a book or other work of art, including the first and last word and all other words except articles, coordinating conjunctions, and short prepositions
* For the call letters of radio or television stations
If it's not on this list, don't cap it. Easy, right?