Well… there are exceptions to the rules (at least, regarding new blocks/entities); with Bukkit it is possible to ‘cheat’ and treat some blocks as different (or combinations of blocks), which has been made easier since we can now attach metadata to blocks and entities (I’m still playing around with this). Spout, which can use Bukkit plugins, has it’s own client wrapper which allows for new blocks and textures without a lot of client-side mucking about…. but it’s still a pain since you have to make sure players use the Spoutcraft client (still seems to have a lot of issues, or at least to me).