There a a multitude.
I’ve heard some guitar builders describe the Telecaster, Stratocaster, Les Paul and SG as “the big four,” and it’s fair to say that most electric guitar designs are derivatives of these four designs, so I’ll focus on those designs.
First, the ergonomic limitations of these designs are most noticeable while standing. For most people, these guitars do not hang in the optimal playing position when standing (which should be somewhat similar to the seated classical position).
With the exception of the SG, the placement of the strap buttons on the upper bout or upper horn results in the guitar hanging under the right armpit or flat across the chest. No other positions are stable. The SG usually has a strap button placed behind the neck heel, which results in superior position, but the design is inherently neck heavy and prone to neck dive, so the neck has to be supported by the fretting hand while playing to a much greater degree. Changing strap button position on the other three designs improves playing position, but the result is unstable. Some very heavy Les Pauls aren’t prone to this, but the extra weight is detrimental in other ways.
In order to tune to pitch, a guitar with a headstock must have the headstock back-angled. The Gibson designs incorporate the back-angle and the result is that Gibson guitars are comparatively very fragile, and prone to headstock breakage. The Fender designs don’t use the back-angle and use string trees instead, which are detrimental to tuning stability. The three a side headstock design used by Gibson results in strings binding at the nut, which again is detrimental to tuning stability.
Removing the headstock solves all of these problems at once.
Another issue with most traditional designs is access to the upper registers. The Les Paul is the worst offender, but the others have their limitations also. The lower horn on a single cutaway or double cutaway guitar impedes wide stretches in the upper registers, even if the horn is minimised and the cutaways are made as deep as possible. A better solution is a body design with no horns at all. The Flying V is a traditional design without horns, but that design has a host of other issues.
The blocky Fender bolt-on neck joint is similarly obtrusive. A set-neck or neck-through construction allows for less obstruction, but it is a weaker mechanical connection than bolt-on construction, more fragile and more difficult to repair. Even then, there are plenty of set-neck or neck-through guitars with poor access to the upper registers, like the PRS Custom 22.
Steinberger GLs don’t have the nicest neck to body joint, but it’s less obtrusive than traditional designs. I’m sure Ned would have updated the heel if he was still designing guitars.
Moreover, the original Steinberger GLs are made of moulded graphite. They are basically indestructible, and are incredibly resilient to environmental changes.
Steinberger bridge designs are also marvellous. The double-ball system results in fast string changes and rock solid tuning stability. The vibrato systems go even further. The S-trem (hinge mechanism) and R-trem (knife-edge nechanism) are both excellent systems, and the Trans-trems are an engineering wonder, if a little difficult to setup and perform maintenance on.
With Gibson’s current financial trouble, I hope the Steinberger brand gets sold to some other company that’s actually interested in realising some of its potential.