Styled Layer Descriptor (SLD) and Symbology Encoding (SE) are two related OGC standards. They have been around for over a decade but I think we can safely say that they have not definitively met their primary purpose - to enable the portrayal of feature data in an interoperable way. The most common complaint with these standards boils down to one thing - a high barrier to entry. This manifests itself in a few ways:
- SLD and SE are XML encodings. While many server-side developers are comfortable dealing with XML, this only represents a small fraction of the market these days. Most developers want nothing to do with XML if they can help it. JSON is a more natural choice for web applications, mobile applications, and many server-side frameworks. CSS is comfortable for many front-end developers. SQL developers (think GeoPackage) want to be working with SQL predominantly. A way past this issue would be to produce a conceptual model that could then be implemented in formats more appropriate for the target environment. The conceptual model could then be implemented a fairly small number of times and then abstracted for the various encodings.
- SLD and SE are fairly complicated. The documents are long and intimidating and compliance is an all-or-nothing affair. It would be helpful to have something akin to "Simple Features" for SLD and SE - a core that only covers the most important and common options. Like most things in software development, the Pareto Principle applies.
- Tools are lacking. There is currently no public executable test suite for the two standards. While GeoServer's SLD and SE support are pretty robust, alternatives are pretty hit-or-miss. If we want people to use these standards, we need the tools that make them easy to use.
On the flip side, SLD and SE also suffer from a lack of extensibility. There are use cases that cannot be handled by SLD and SE out of the box and without an extension mechanism, there is no clear way for a developer to introduce interoperable solutions. This means that complex portrayals (think MIL-STD-2525) are a non-starter.
When a solution is not ideal for either the low end (lots of potential users) or the high end (lots of potential funding), all that is left is the skinny middle and it isn't enough. This is solvable if we have the will. That phrase is key - if we have the will. The solution is not going to fall from the sky. It is going to take effort and leadership from a number of organizations over the course of more than a year. OGC appears to be willing to take the lead here but the effort requires community support to be successful. There will be opportunities to participate and I hope that you will find a way to do so.
 The "TENET report" enumerates several limitations of the OGC SE, including hierarchical symbols, multiple delineations, and pivot points.
 Total tangent here. I first drafted this post on 9/13/17 and happened to include this phrase which comes from a Grant Hart song of the same name. That very night he passed away (unexpectedly, at least to me). RIP Grant.