First of all, those recessed handles on the new Laney VH100R's are fucking AWESOME, so there's a practicality thing
Another essential IMO is an FX loop (solid-state buffered and serial please, for maximum transparency) that can be bypassed via the footswitch, and also have an optional volume boost for when it's enabled (the practical application of that for me is for soloing, so I can have a delay always-on in the loop, and then kick it and the volume boost in with one step
) Some amps have two master volumes you can switch between, and I wouldn't mind that either, but I'd prefer to have the option to have the increase automatically kick in when the loop is enabled!
BUILT-IN TUBE SCREAMER
Not too many mini-toggles or other options, and I especially can't stand amps that have two gain controls for one channel (VHT stuff, for example), too confusing IMO
A bias-adjusting pot accessible without removing the headshell from the chassis is always a plus
That's all that comes to mind right now!
EDIT: And wow, I pretty much disagree with everything in Notuern's post
As far as tone-stack goes, If it ain't broke, don't fix it IMO, and more tubes for independent channels means more heat and more power consumption! (strain on the transformer). And the complexity of a built-in power soak feature could be forgone in favor of a half-power switch methinks