This conversation can actually go in two directions; Advancements in machine quality and capability, and advancements toward the reprap dream of self-replication.

On the self-replication front, progress has all but stopped. Initial progress was made by trying to reduce machined part counts. This was semi successful but not much has happened since then. Most people would rather build a printer that works well with prefab hardware than work towards self-replication. Any progress made in this area is mainly by companies working on metal printing technologies and multi material techniques. Once the technology is refined enough to print PCBs, then we will see drastic progress in my opinion.

On the machine quality and capability front, I think progress is steady. There will always be that desire to make your own printer better, and that will bring about occasional breakthroughs. But, like people have said above, there are several things limiting faster paced advancement. The largest one is cost in my opinion. Repraps are almost exclusively used by tinkerers and DIYers who use 3D printing as a hobby. With the minimum cost to build a printer around $500 ( just an estimate), that stacks up to be a fairly expensive hobby! Especially if someone was to do the experimenting, building, redesigning, rebuilding that is required for any substantial advancement and innovation. The more people that join the community the faster things will move but money will always be an issue for reprap progress.

documentation is also a point that I agree with but I won't restate what has been discussed above.