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).
Still, you would think there would be an easier way to find good documentation. Given that Solr is all about searching and finding things. Perhaps, there is an opportunity to innovate in this space and use Solr to help people with finding information about Solr. Dogfooding and and all that.
The contest runs for another 3 weeks, so there is enough time to add more suggestions, vote for the most interesting ones or - even - suggest solutions to the ones that already got a lot of attention. Please join in. And remember to login, if you are making a suggestion and want to win a copy of my book.