Installable from GitHub, maven, sonatype or elasticsearch.org
Depends on separate ZooKeeper server
Only ElasticSearch nodes
Automatic node discovery
internal Zen Discovery or ZooKeeper
The partition without a ZooKeeper quorum will stop accepting indexing requests or cluster state changes, while the partition with a quorum continues to function.
Partitioned clusters can diverge unless discovery.zen.minimum_master_nodes set to at least N/2+1, where N is the size of the cluster. If configured correctly, the partition without a quorum will stop operating, while the other continues to work. See this
If all nodes storing a shard and its replicas fail, client requests will fail, unless requests are made with the shards.tolerant=true parameter, in which case partial results are retuned from the available shards.
Automatic leader election
Automatic shard rebalancing
it can be machine, rack, availability zone, and/or data center aware. Arbitrary tags can be assigned to nodes and it can be configured to not assign the same shard and its replicates on a node with the same tags.
Change # of shards
specified at index-creation time, with command-line param -DnumShards=n. Can be increased by splitting an existing shard (SOLR-3755). Cannot be lowered. Additional replicas can be created.
each index has 5 shards by default. Number of primary shards cannot be changed once the index is created. Replicas can be increased anytime.
Relocate shards and replicas
can be done by creating a shard replicate on the desired node and then removing the shard from the source node
can move shards and replicas to any node in the cluster on demand
Control shard routing
with some config changes
Indexing requests are synchronous with replication. A indexing request won't return until all replicas respond. No check for downed replicas. They will catch up when they recover. When new replicas are added, they won't start accepting and responding to requests until they are finished replicating the index.
Replication between nodes is synchronous by default, thus ES is consistent by default, but it can be set to asynchronous on a per document indexing basis. Index writes can be configured to fail is there are not sufficient active shard replicas. The default is quorum, but all or one are also available.
As a number of folks point out in the discussion below, feature comparisons are inherently shallow and only go so far. I think they serve a purpose, but shouldn't be taken to be the last word on these 2 fantastic search products.
If you're running a smallish site and need search features without fancy bells-and-whistles, I think you'll be very happy with either Solr or ElasticSearch.
I've found ElasticSearch to be friendlier to teams which are used to REST APIs, JSON etc and don't have a Java background. If you're planning a large installation that requires running distributed search instances, I suspect you're also going to be happier with ElasticSearch.
As Matt Weber points out below, ElasticSearch was built to be distributed from the ground up, not tacked on as an 'afterthought' like it was with Solr. This is totally evident when examining the design and architecture of the 2 products, and also when browsing the source code.
My other sites may be of interest if you're new to Lucene, Solr and ElasticSearch: