Global effects bypass
One feature I've always pined for would be a simple plugin bypass which automatically deactivates all the plugins.
Plugins are great, but they take up valuable CPU cycles, can cause major system instability and can really destroy productivity. That said, if you like plugins in your mix, its usually very tempting to apply them long-time before mix-down.
Plugins evidently can already be bypassed (for example, having them off when recording) or you could go and activate/deactive them yourself at will.. however I can't help but think how handy a global bypass button would be.