RCDB is hard to improve and is by far the most comprehensive record of coasters in existence but there's one thing that annoys me. It is often difficult to to find more obscure coasters that you cannot remember the name of or the name of the park they are in. The tree view method is okay but unless you have an amazing knowledge of the geography of each individual country, it is somewhat pointless.
I think that to ease this problem, the tags system used to search for coasters should be more comprehensive. For a start, more informal tags that enthusiasts usually attach to coasters could be used in conjunction with the usual suspended/sitdown, steel/wood style tags. These might include multi-looper, hyper coaster, jet coaster, twister coaster, terrain or even having the coasters categorised by their track colour. It might sound a bit daft but if you've only seen a photo of a coaster, its colour is pretty much the best thing you've got to go on outside of track style.
Another nice addition to the search features would be the type of coasters that you don't want to show up on the list such as Kiddie Coasters if you're looking for an obscure large steel coaster. This would stop your search from being populated by a million Wacky Worms and Wild Mice that you can't tell are what they are until you click on them. At the moment there's no way of preventing this as although there is a Kiddie Coaster tag, they still fall under the same categories as the coaster you are searching for.
None of this is in any way a complaint as RCDB as it stands is still excellent but there are small improvements that could be made