Windows has no problems with lowercase letters

Unfortunately it does. Try to create a file "FOO.TXT", and then create a new file "foo.txt". You will find that the second file "replaces" the first one. The reason is that Windows internally does not distinguish between lower and upper case. Well, this applies to Windows until XP. I don't know whether this has been fixed in Vista.

and fewer problems than unix with the dash

Partially true. Strictly speaking, Unix has no problem with dash, but you likely mean that the vast majority of Unix command line tools use the dash to denote switches, while the majority of Windows command line tools use a slash to denote a switch. Since there are still quite a few Windows utilities which also use a hyphen to denote a command line switch, you should avoid on Windows files starting with either a hyphen or a slash, while on Unix you will have a happier life if you don't have files starting with a hyphen, and a slash is ruled out as valid character anyway.

Underscores, AFIK, are no problems in either OS. In Windows, there are some characters which you should avoid, since they have special meaning to the shell: caret (^), percent (%), double quotes ("), parentheses. Caret and percent are fine on Unix, but the others are better avoided there too. While Unix itself is tolerant on most characters, many of the shells on Unix have their own special characters, such as a tilde (~) on the starting position, or a $ sign.

On Unix, I try to avoid characters which are special to the most common shells (bash, zsh, tcsh), not because it would be strictly necessary, but because it makes life easier. Given that at least tcsh is available on Windows as well - though it seems to be much less in use than on Unix -, I avoid these characters on Windows too.