It's not so much the tools/engine you use as it is how you design and implement your game... RealmCrafter only gives you the foundation to build your game and whether or not you try to get the WoW affect from your game is up to you, not the tools.
No. Not at all. What you can design and implement is directly constrained by the tools and engines selected, or vice versa.
By "WoW knockoff", I mean that if he wants to create something truly unique or with high reqs functionality- and capability-wise, not look-and-feel-wise, there are inherent limitations based on the current state of the codebase of RC.
There will be functionality or reqs you can kinda sorta creatively twist into RC to do without much coding, and some that require access to and modifying source, and some just really difficult to the point that alternate approaches would be warranted. If there are many of the latter, then RC is not suitable.
It's not about bashing RC, but the reality of any toolset questions a developer needs to ask him/herself before adopting...
Also, Demented, you'll want to gauge the community that surrounds any toolset you use, because you would like to have a community you can lean on for help.
PS: One of the difficult part of RC is the closed environment. For example, let's say I wanted to know about realistic limits of players per server on RC Pro, or if one could successfully implement a physics-heavy, realistic jousting MMO, I have to buy RC Pro before I can ask users on the locked, licensee forums. That's like a contractor only giving you references after you sign the contract...