There are many comparisons between Apache Solr and ElasticSearch available, so I'll reference those I found most useful myself, i.e. covering the most important aspects:
Bob Yoplait already linked kimchy's answer to ElasticSearch, Sphinx, Lucene, Solr, Xapian. Which fits for which usage?, which summarizes the reasons why he went ahead and created ElasticSearch, which in his opinion provides a much superior distributed model and ease of use in comparison to Solr.
BOB的文章列举了他选择 ElasticSearch的原因： 与Solr比较， ElasticSearch 提供了非常棒的分布式模型，以及易于使用。
Ryan Sonnek's Realtime Search: Solr vs Elasticsearch provides an insightful analysis/comparison and explains why he switched from Solr to ElasticSeach, despite being a happy Solr user already - he summarizes this as follows:
Ryan 这篇文章（ solv vs elasticsearch) 做了深入的对比，解释了他告别Solr 投入 ElasticSearch怀抱的原因。 尽管他已经是一个很不错的Solr用户，不过还是有这样的话：
Solr may be the weapon of choice when building standard search applications, but Elasticsearch takes it to the next level with an architecture for creating modern realtime search applications. Percolation is an exciting and innovative feature that singlehandedly blows Solr right out of the water. Elasticsearch is scalable, speedy and a dream to integrate with. Adios Solr, it was nice knowing you.
Solr 只是建立“标准检索应用”的有力武器，但是 Elasticsearch 则更上一层楼： 建立最新潮的 “实时检索应用”。 它凭借 新特性Percolation 把Solr 彻底打败，而且有高度的扩展性，高速度，它就是我梦寐以求的。 Solr，认识你很高兴，不过还是再见吧～
ElasticSearch 的作者 Shay Banon says:
“NRT commit” solves it for the single node case, but in the distributed case, you really need a distributed model that supports it. Solr, currently, does not, while elasticsearch does.
NRT commit (接近于实时提交）只适用于单节点的情况。对于分布式的情况则不适合。 现在看来，Solr不支持分布式。 而 ElasticSearch 可以。