The current LL/OS mesh, armature, morphs, UV maps, and default diffuse and bump maps have many problems. They're so bad that high poly rigged mesh attachments and their textures, hugely wasteful of viewer and server resources, are the norm, used to cover up the system avatar.
The system avatar was developed in Maya, which is expensive, proprietary, and lacks inexpensive ways to build appropriate plugins. In 2005 it was the appropriate choice. In 2020 Blender is a better choice.
The system mesh topology is not well suited for animating or morphing, and not up to current standards for human, humanoid, or animal topology. Some parts should be eliminated: skirt, hair, separate eyelashes. The body is divided into head, upper and lower sections in such a way that normals (face direction) don't blend and seams are awkwardly visible.
The armature is not fitted correctly to the mesh to make auto-weighting possible in 3D modeling software. Some bones are redundant, other bones normally found in current armatures need to be added. The bones are not named consistently, rendering many tools in 3D software useless, and vertex weighting highly inefficient. The bones do not have left-right symmetry in all cases, and the bone head and bone tail positions for connected bones don't line up in all cases. The left-right order in the hierarchy is not consistent - left bones are listed first in most cases, except for the legs and feet, where right bones are listed first. The collision volumes, which are used to rig mesh attachments that will respond to the (morph) Appearance sliders, were not originally meant for rigging and should be eliminated, to be replaced by current standards in armature construction. (the so called "Bento" bones, added in 2016, have much better design for the Appearance sliders, and are left-right symmetrical)
Viewer software limits the number of bone vertex groups that can be uploaded with rigged mesh to 110, even though the current count is 159 bones in the LL/OS armature.
The inworld morphs for the Appearance sliders and inworld default animations should be adjusted so they better match the inworld mesh rigging, so that artists making mesh or prim attachments can get them to work more predictably.
The UV maps for the mesh need to be updated. The seams are too visible, and the UV maps themselves are not up to current standards: they should be symmetrical, so that artists can design diffuse, normal and specular maps more efficiently.
The diffuse maps and bump maps for inworld skin, eye and hair sliders should be redesigned with better artwork that takes advantage of, and doesn't duplicate the shadowing now available in current viewers.