I think three of the biggest frustrations and oft-requested enhancements in the vast majority of media servers are as follows:
1. Album Artist. If the server only reads the track Artist then your Artist menu gets clogged up with hundreds of minor, contributing artists that are only in your collection simply because they appear on an obscure compilation album, and you would never really want to search for them explicitly. Also, albums by a specific artist get split if one or more tracks have a contributing artist that appears in the Artist tag.
2. DiskNumber/Part of Set/Volume. There are not many servers at all that can handle multi-volume/boxed sets. Although the uPNP spec says that DiskNumber is be a digit, it would actually be more useful to have it as a string so that you can name your volumes, e.g. "Original Album" and "Bonus Tracks" for example, or in a boxed set of world music where each CD is themed after a country, the DiskNumber/Part ofSet could say "Africa", "Austria", "Australia", etc. If there is no consideration for boxed sets, then you either have one album with dozens of tracks in it, or your main album menu is cluttered with each individual CD whereas you'd just want to see one entry for the entire boxed set.
3. Sorting albums (within a particular artist) by year rather than alphabetically.
Ideally, here are some basic views that I feel should be part of any DLNA server to cater for most peoples' tastes.
1. Album Artist ==> Album (sorted by Year) ==> PartofSet (skip this level if there is only one option) ==> Track (sorted by TrackNbr)
2. Artist ==> Album (sorted by either Title or Year) ==> Track
3. Album (sorted by Title) ==> PartofSet ==> Track
4. Genre ==> Album (sorted by Title) ==> PartofSet ==> Track
5. Genre ==> Album Artist ==> Album (sorted by Year) ==> PartofSet ==> Track
Using Album Artist means that you can easily find compilation albums by going the artist called Various Artists, and your main artist menu is uncluttered.
Artist and Album levels should also have an option to generate an A-Z index first before drilling down into the item.
As the OP has stated, many people also like to browse by Year or Decade as a top level item.
For me in particular, I also have a Grouping tag that always comes between the Album Artist and Album levels, and this contains values such as Studio Albums, 7" Singles, 12" Singles, CD Singles, Compilations, Live Albums. So that when I go into a particular artist, especially if I have LOADS of items by that artist, I can further break it down by the type of album rather than having them all mixed up together.
Images for folders are not part of the UPnP/DLNA standard.
True, but then again neither is transcoding to anything other than WAV and MP3, nor catering for the limitations of various clients by having separate profiles that supply customised things!

Some media players (e.g. the WD TV Live) already understand images for folders because in graphical browse mode there is no text to say what the folder is. If the server doesn't supply an image then you just get a screenful of generic folder images with nothing to distinguish them until you roll over them.