Not really a big problem but the tripod foot is too small. I like to hold the lens by the foot. I've read that some find the manual focus ring too narrow but I have piano fingers and it's perfect for me.
I've now handled and used both versions. The Sport is beautiful, beautifully engineered and heavy. The weather sealing is also a big bonus for the nervy photographer living in rainy Britain. The C has a much cheaper build quality with more plastic but for me is like handling a 70-200 so could be used handheld all day. The C is also much, much cheaper.
Not sure they are really problems as such. If you assess it purely on the price and not comparing it to prime lenses in terms of quality I think it's phenomenal value. The biggest problem I had was whether to choose the Sport or the Contemporary.
One tip is to keep checking
www.camerapricebuster.co.uk. It was retailing everywhere at £899 and I got mine for £849 and it was as low as £799. Digital Depot seems to do deals for weekends only. That was less than a month ago. Prices have probably moved a little since.