Here is a list of the common mistakes I came across: whether instead of rather. This one was new to me. And horrific, if I may add. Then instead of than. (i.e, rather then instead of rather than, or I like that shoe better then instead of I like that shoe better than) This mistake always has me scratching my head because I just don't get how people (especially people who call themselves writers) get these two mixed up. Another one I kept coming across was Your instead of You're. Your bag looks really expensive. Not you're bag looks really expensive. Your is a possessive adjective (i.e., is this your bag? I like your watch.) You're is the contraction of "you are" (i.e., you're going with us. You're singing too loudly.) Also notice how the use of you're is mostly followed by verbs in the present participle ing ending. A few other mistakes I kept seeing over and over from the same writer, was the use of where instead of were, there instead of their, and affect instead of effect.
Let's be honest, I read with an editor's eye, so, perhaps, the chances that I would very easily catch things the average reader may not, is a lot higher. But be that as it may, you can never underestimate the writing/reading abilities of your readership. Put a little effort into the way you present yourself to the world, or hire a good proofreader to do that for you. Either way, never send out something you've drafted without giving it at least 3 read-overs. Impressions are long-lasting and the last thing you want is for your readers to be so focused and turned off by your spelling errors, wrong use of grammar tenses, and misplaced punctuations, that they miss the larger purpose of the text - the message you're trying to drive home.