The idea is to have a proper and worthy replacement for the Java version
Some of the content in the Java version wasn't that important IMHO, so we try to focus on more important things atm... but even when the most important features are finally implemented, is it really better if we focus on some of the smaller stuff of the Java version (e.g. curtains, banisters, a rolling pin etc) instead of working on more useful features like the ability to place items like tools & weapons in the world, for example?
In fact we're running out of money (because all updates for the new version were mainly targeted at the existing community and this barely results in any new sales), so we finally have to bring the new version into a state where it could be considered "fully playable" (at least to the same degree as the Java version). The current situation is quite problematic, because we barely sell any copies of the game (considering the store page on Steam still advertises the old and dusty Java version). We all agree that having more plants, objects or new building materials would be really nice (and these things are indeed on our to-do list), but first we have to focus on the key features (biomes, caves, hostiles etc).
Once the Java version had been replaced with the new version, we can finally focus on adding smaller features and other things as well 
ability to put posters on walls [...] things like flowing water
Posters were added with the latest update
In fact they're even more versatile compared to the Java version, since you can now paint posters, and there is also a powerful new "decal" poster type available (which adapts to uneven surfaces, like on this screenshot)^^
Flowing water is also in the game already (it was added with the world generation update back then) 