My kneejerk reaction is; shouldn't WoT fans be thrilled that the series will be finished at all?
At the end of the day, I know I'd prefer a book that took as much time as it needed to be finished, and was the best it could be. That said, at almost 20 I know some people have waited my whole lifetime for this, so a little impatience is understandable. I sometimes wish I could just read it now, but it's not something I'd go saying all over the Internet.
We don't always think before we type. Should they pause to think about it, I'm sure the 'jerks' in question would realise Brandon's been almost superhuman in finding a balance between the two, putting a phenomenal amount of hard work into very little time. I don't see how anyone could ask more from the guy! The sheer scope of WoT is incredible. I wouldn't have a clue where to start in Brandon's shoes. 
Well, a lot of the negative reaction from WoT fans was after the announcement of the split, but before the release of TGS, so many of them had not yet read anything of Brandon's or discovered many of the reasons we already trusted him as an author. There was definitely a perception that the split was a commercial thing to make more money by filling out the book. (which will seem ridiculous to anyone who's read TGS or TofM, but that's what people were worried about)
As for the fans now who think that Brandon should be writing faster and only working on AMoL, it's probably just deep ignorance of the way the writing process works for an author, especially someone finishing up a large series like the Wheel of Time. There are many people who just don't consider that Brandon will need to continually re-read the books to stay anywhere near as familiar with the series as Robert Jordan was, and they probably don't even understand that writing fiction involves a lot of false starts, writing group and alpha-reader feedback, re-writing, and more feedback. Many people just think you sit down and bang out however many hundreds of thousands of words and you're done, lol.