Mgccl's Ivory Towerso tired

Around a year ago, I had a vision of a problem database(it's like wikipedia for problems). I made a prototype on top of Drupal.
It was nice, but there are many problems, one of them is to extend the attribute of every type of problem. This is not a difficult task. Drupal's CCK fixed this up rather nicely.
The relations between every problem are hard to map out. Soon I found Drupal's limit, and abandoned it for the project.

I try to write my own program for it. I found in order to make each type of problem have flexible set of attributes, I have to re-implement Drupal's CCK in my program. There must be a better way. In the end I'm convinced that a relational database doesn't suit my need, because of following reasons.
1. Each set of problem can have different set of properties. Some doesn't present in other .
2. Relational database are horrible for graph structures. In fact, relational database are horrible for any structure that can't be presented as a table naturally.

Then I found CouchDB and MongoDB. Both are document databases, it address problem 1 by schema free design. Problem 2 it cannot take care naturally, thus I have to write the code to handle the graph structures. It's still going to be slow.

Just today, I found there are graph databases. Like neo4j. neo4j store everything as graphs. The relations are mapped out as graph in the database. It is also schema free.
It addressed both problem 1 and 2. Awesome.

Looks like I got everything to build the thing I wanted. Hmm... The technology is as mature as I need.