Quoted from Thermionic:

Bullet points:
*No credit dots.
*Everything functions as it should; displays are bright and clear with no dim/weak segments or burn-in. Flippers have been rebuilt with fresh coil sleeves and coil stops, and EOS switch properly adjusted (at minimum). Switches should be appropriately adjusted.
*Game is cleaned inside and out, any cabinet blemishes are fixed; playfield cleaned, major blemishes appropriately repaired, and preferably freshly waxed. Plastic ramps cleaned/polished and haze-free. Mylar/Cliffy protectors installed in high-impact areas.
*All-new incandescent light bulbs; games with stock LEDs don’t necessarily need total replacement, but all bulbs should be bright and flicker-free. All bulbs should match in brightness, color temp.
*All plastics should be clear and free of cracks/breakage.
*If game is >5 years-old, all rubbers should have been replaced; for newer games, all rubbers should at least be in new or nearly new condition.
*Any known factory weak points should have been addressed/upgraded. For example, underspec’ed connectors, diode bridges, and ground planes should be fixed/modded; on-board alkaline battery holders should be relocated off the board (or NVRAM should be installed) and any corrosion from prior leaking should be appropriately repaired.
*Any suboptimal hacks should be redone consistent with appropriate electronics construction standards.
*Stainless parts should have been cleaned/polished and tarnish-free. Painted parts are scratch and chip free.
*Glass should be scratch-free or replaced.
Basically you should get a game which functions as close to new as possible, and any “patina” should be unintrusive aesthetically and functionally. Done right, a fully shopped/serviced game should meet or even exceed NIB durability and reliability.