I'm not sure if this is a hardware or software issue. I just started using AutoCAD 2015 which is very different from 2014. At the same time, I also got a new computer containing a low-end, unsupported graphics card. I requested a graphics and RAM upgrade but haven't received it yet. I turned off hardware acceleration and made sure display lineweights was turned off. Yet, my linework is still displayed as if weights are turned on. I'm not sure if this is just the way the new version is supposed to look and I have to get used to it, if there's a setting I'm not aware of that will fix it, or if my expected hardware upgrade will fix it.