The information below is from a site appropriately called 'Skills you need'.
Overusing Capitals is Rude
WRITING ENTIRELY IN BLOCK CAPITALS IS SHOUTING, and it’s rude.
We’ve all done it: left the Caps Lock on while typing. But in email etiquette, online chats and/or forum posts, writing in capitals is the online equivalent of shouting. It’s rude, so best not to do it unless you really do want to shout at someone. Even then, consider whether you’d really do it if that person was in front of you, and also whether it will get you anywhere.
Although it’s usually best to avoid writing in capitals, it can be useful to write odd words in capitals to give them emphasis. HELP! You're going to LOVE the surprise.
It’s also much harder to read block capitals as all the letters are the same height, so you will make your point much more easily if you use lower case.
