My personal pet peeve: when writers gloss over the part I am looking up the help for. I hate when I read a section and it says "this fight is easy, just kill the monsters" or something like that.
No kidding. This is pretty much the one reason GameFAQs has always annoyed me, many of their guides are just like this and assume everyone's either an expert at the game or just plain lucky. It really, really felt annoying when trying to get Mario Galaxy 2's Green Stars and other similar stuff.
Here's a tip that wasn't mentioned though...
If you're not writing your guide as an FAQ for GameFAQs or perhaps IGN, you don't need to limit it to text only. If you're writing it for a website (maybe one you own/are staff are), try and use images and perhaps video clips where relevant to help people out in regards to things like where to aim or where to go or maze solutions or similar puzzles. It's always bugged me when I see a not GameFAQs site host their own walkthroughs and then not use the chance to use other media to make the instructions clearer to the player.