I'm both a 2D/3D disigner and programmer so... It's a bit hard for me to evaluate Unity from only a non-coder point of view. Unity is easy to understand but for moving around it's better to have a bit of 3D knowlegde. For making things programming it's a must, it's not like Visionaire. It's a general engine, so you can see good coded and optimized games and poor coded games with it. Adeventure games done with Unity that I can remember are Larry, Space venture, Jane Jensen's Moebius and the GK remake, Broken Sword 5 and AR-K.
Unity has its strong points on portability (exporting to mobile, windows, mac and linux... AND working on windows, mac, -linux?-) and easyness of work from a programmer point of view. You can do a lots of things with little scripts here and there and in the last version they added 2D tools. But it's not an adventure game engine (you can program on top of it whatever type of game you want.) and it's not a non-coder engine...
Their comunity is wide, and they have an asset store, where you can buy prepared 3D models (characters, enviorments...), plugins (for doing 2D, in-apps, accesing databases, blabla....), editor extensions, and whole kits for starting a type of game (runners, space sim,...). One of those kits/editors it's Adventure Creator that @Rocco linked up there (watch the videotutorials!).
Adventure Creator it's a great plugin... It's more focused on 3D and 2.5D games, but the creator added 2D support recently. It comes with point&click/first person/direct control interfaces, save system, inventory, localization, pathfinding (its own version and Unity's version), and the interactions are made with actions list like Visionaire.
The menu and dialogue system could be better. Right now it's a mess for doing complicated dialog trees. The menu system has recently updated but I didn't see this last version yet. Pathfinding and character scaling for pure 2D games (painted backgrounds) are hard to do without 3D knowledge. Anyway, it's a really good kit, but I think its real strength comes from its author, who listen to every request at the Unity forums and it's delivering fast updates (something like one per week!) adding requests and new things to the kit. And the source code its included so you are able to change whatever you need add more actions, etc.
Also there are other adventure game kits on development like http://forum.unity3d.com/threads/202293-Adventure-Game-Frame...
Unity is a good multiplataform game engine but remember it needs 3D knowledge and real programming work or a 3rd party kit like Adventure Creator to make an adventure gamel. And Visionaire is right now a real world-tested product for doing 2D adventure games, but only for Windows and Mac (iOS in the future), and I think its development is a bit slow..