The most popular suggestion so far is Better Documentation. This is both easy to predict and a bit sad. From my own experience, there is quite a bit of documentation about Solr on the web, but it might be a little hard to find. Some of the best stuff actually hides in videos and slideshows, so may not even be easily visible to Google. And then, there are books - also not indexable (see relevant suggestion). Some of those books are quite comprehensive. And, of course, for the advanced material, the source code is self-documenting on the most basic level, yet again not something Google can search effectively (use other search engines for that).
In collaboration with Packt Publishing and to celebrate the release of my new book Instant Apache Solr for Indexing Data How-to, we are organizing a contest to collect Solr Usability ideas.
I had to set up Apache Solr 4 on Windows as a service using Jetty container. The following is the documentation on how to do it. I am not saying that this is the best way to get it to work. But it is one way that works and seems to be more recent and more comprehensive than the other approaches I found.
It is a great moment. After many months of work, my book is finally published and is available from multiple sources. It is called Instant Apache Solr for Indexing Data How-to and it has been published by the Packt Publishing.
There is a number of books published on Solr, but I feel that mine is different. Most of the books try to cover as much of Solr as possible and have a reference-style approach to explaining what different Solr components do. This is useful but - because Solr is so large - it is easy to get over-saturated with all the information and still have no idea of how to put a good Solr setup together.