Having been a part of the Baldurs Gate 1 and 2, Oblivion, and Fallout 3 mod communities I have to say the type of talent sitting out there chomping at the bit to mod an MMO is incalculable.
If Turbine were to open set up a "Trial" Program where they screened applicants and gave them limited access to code, they could IMO make the best MMO ever created, toppling WoW.
Here's how:
Open to the Community at large:
The "3D Modeling" tools available and let the community create new 3D Models to be implemented. There would be no real harm from this as they cant change any "code" related to actual gameplay. However I would almost gauruntee in a few months Turbine would have thousands of new skins and models available for new monters. Turbine could set "Project" goals like "We need the following 3D Models: Bears, Owlbears, ThriKeen, etc" that would direct the community to create whats needed.
Open to Verified Applicants:
Make the "Quest Scripting" tools available, how many new quests would be produced in a short amount of time? You could, in theory set up a server similar to Lama where users could then try out and rate the new quests and only move the highest rated quests to the "live" servers. Once again, the I'll effects on the live servers would be next to none as the exploit quests could be weeded out (kill 1 kobold get 100,000xp) and only the "Cream" would be made live.
Open to Verified and Screened Applicants:
General code with the requirements of signing a NDA and requiring Drivers License or Passport identity verification and agreeing to certain terms and conditions. Violations of said terms and conditions would terminate the users access.
Now, that may seem like a lot of "Extra" work on the part of the developers to implement, but when you calculate the amount of "free man hours" gained and the potential word of mouth advertising for being the "First Community Source" MMO I personally would sign up for the quest scripting and be churning those out on a regular basis!