In addition to what the others mentioned, barrel length is important when using the 5.56mm round, because muzzle velocity (which relates directly to lethality) depends heavily on barrel length. Much of the criticism of 5.56mm lethality comes from the prolific use of shorter barreled M4 carbines over the full length M16.
A common shortcoming of the bullpup design is that it is right hand only: If you fire something like the Steyr or SA80 from the left shoulder, the brass will eject straight into your face. The Steyr can be made left handed by changing a few parts, while the French FA MAS can do so by simply rotating the bolt and switching the cheek pad to the other side (covering the ejection port on one side and exposing it on the other) or so I've heard.
While certainly not perfect, the current version of the SA80, the L85A2, is a vastly improved weapon, and generally troops are pleased with it. Some of the problems were due to the design and the bullpup configuration, others were down to horrible management of the whole programme.
There's also a round called the 6.5mm Grendel with superior long-range performance. And work is still being done on advanced munitions types: Caseless and case telescoped rounds.
Barrett has now moved on to the REC-7: Basically the same weapon, but now with a gas piston system and some minor changes.