Aside from hunting for bugs and balance issues, the private testers have also been giving their input on the game itself and features that they'd like to see implemented. From something as simple as adding signs to the front of buildings for identification (a Blacksmith sign, Inn sign etc.) to making the HUD more user-friendly, their suggestions often really improve certain aspects of the game that I've overlooked.
For example, check out this older screenshot:

Screenshot from a much older version...
And compare it with this one:

An image from the latest compile (v.28)
Notice anything different? Stackable items such as potions now have an overlay that displays the number of items in the stack. Additionally, shopkeepers now sell potions in greater quanities so that you don't have to buy one at a time.
You'll also notice the bow with the blue underlay. And underlay like that denotes that an item is "Enchanted" (the lowest level of rarity after normal items). If the item were "Rare", the underlay would be green. This new feature suggested by the testers helps to more easily identify valuable items without having to click on them.
Pretty simple stuff, but every little bit contributes to what is becoming a very polished product. Without private testing the initial release of the game would have been raw, unbalanced, and buggy. However, thanks to the dedication of the testers, you'll be fighting monsters instead of glitches on the first day.
Gimme.