Well, again, speaking for me personally:

• Better food in lounges (AY or BA GC level, say.. I'm realistic enough not to expect anything too fabulous), more space and more diligent staff. At least one dessert option all day other than sugary biscuits. Better quality soup (I don't want hot dogs, thanks OSL).
• Perhaps free choice of one food and drink item from the BoB menu (I realise this totally erodes the distinction between Plus and Go on short haul flights, although currently I can get a sparkling water for free but not a chocolate bar which costs exactly the same)
• Properly enforced priority boarding
• Better handling of short/rebooked connections. If I can walk at a normal pace to the connecting gate and arrive before boarding has even started, then I should not have been offloaded, and if you had someone waiting to escort me and the other connecting pax you wouldn't have to pay EU 261 for them, and my blood pressure would be lower.
• Actual priority for Diamond/Gold on the helpline. When IRROPS strike one sits and waits for 15 minutes before being chucked off, along with the rest. In addition, the Diamond helpline could be differentiated from Gold, much like BA's GGL - they should be able to answer most calls within a couple of rings, even when things are busy, and have added discretion in service recovery situations. I appreciate it's easier to qualify for Diamond than BA GGL, so this analogy is not intended to be exact.
• Recognition onboard, on flights that are long enough. It's wooden and not very in keeping with Janteloven but being told "thanks for your loyalty, if there's anything more we can get for you, please just ask" gives one an opportunity to raise issues.

These are all things which would benefit my daily routine rather than some occasional upgrade. The food/drink ones would be a net cost I guess; boarding and connections would not seem to be very costly; the helpline and recognition stuff would require training and additional decision-taking powers delegated to individual frontline staff.

Anyway, they would all mean more to me than some vouchers.