Some shards, especially east coast ones, have extremely crowded populations. Being on them is almost like being in a different game, compared to the lower population ones.
That said, there are two special ruleset shards.
1. Siege Perilous: Has a different skill gain system, does not have Trammel, all facets that are there have PvP ruleset, certain things cost 3 times as much, NPCs don't buy from players, recall and the equivalent spells don't work (Gate Travel does, though), Blessed items are extremely limited (but players can bless one item each), and insurance doesn't exist. Players limited to 1 character per account on the shard.
2. Mugen: As Siege Perilous, except has a Trammel Facet that is the PvP ruleset.
3. There is also the Test Center shards (TC). Test Centers are occasionally wiped, but you can set skills (to recreate to an extent) characters elsewhere, and do other experimentation there. They exist both to test new content for the game, and to let players test crazy ideas to see if they'll work, before spending the time and effort to try them on another shard.