I think the better way to do it is to have a "full respec" and a "slot respec". Both of which would consume a standard respect, but the slot one would only affect slots, so no re-picking powers, no resetting trays, etc.
The reason I think this is a better approach is that if we have a 1 slot respec, people will start asking for a 2 slot respec, and this will continue to a certain point. I doubt people would want a 50 slot respec, but I could see possible 10 slots being wanted.