This is a very difficult question to answer. It is very difficult to determine how different and how dangerous chemical weapons would need to be in order to warrant being classified under WMD.
These weapons could be said to be more dangerous than many conventional weapons. Poison gas could spread across an entire battlefield whereas conventional munitions have a much more localized effect. Chemical weapons ban be made potent enough that only a very little bit can kill, thus making them more likely to kill (as opposed to wounding) than conventional weapons.
As for the difference between the two, there does not seem to be a great difference. It is surely horrible to breathe in a chemical weapon. However, this surely cannot be worse than being burned by napalm or "simply" being shot by a conventional bullet in a way that is very painful but not immediately fatal.
Therefore, it seems appropriate to say that chemical weapons are not that different, but may be dangerous enough to be classified as WMD.