He was a much better story teller than writer, IMO. In fact, it's not that RJ was so poor a writer (though he lacked style - his prose is no more than a story telling vehicle and had little beauty or interest of its own), but that his writing on the whole was old fashioned in style, pace and devices, and that irritates many, especially those who read and enjoy mostly the faster paced and more elliptic "modern" novels. TV has completely changed the way we perceive and expect story telling. RJ still wrote like writers wrote when their readers had the patience and desire to learn all the little details.
Another problem is that RJ pushed the novel format a little too far. It's debatable whether any story should go on at such length that a whole volume becomes like chapters or at least parts of the big "novel" the whole series is in the end. It's part of the game that some chapters have to be slow, set things up - especially before the closure of an act, and at the beginning of another (this is what happened with LOC, COT/WH etc). When the story is so detailed and huge that set up chapters turn into set up volumes, forcibly the patience of many readers will get tried a lot. A massive challenge, forcibly again, is that RJ could not do what virtually all novelists do, which is to complete the novel from start to finish and look back, polish the structure, shift things around, and do massive cuts. Had RJ finished WOT before publishing anything, of course he would have fixed a great deal of things and solved many problems.

Ironically, Team Jordan went and made the same mistake, and if RJ can be said to have been overambitious and having run into problems controlling his story, TJ is far less excusable, because they could realistically have decided to finish the whole thing, at least in draft, before stepping back, looking at it, solve all the problems and consider the best way to divide in two or three a finished story. TOM as it stands is by a very long shot a worse novel than COT, even though with all the action and resolutions it's more exciting to long time readers of WOT. Literary speaking, it's still a complete mess. It makes COT look well structured and balanced, which COT is not!