One of the issues of 3e was that boss battles were very swingy. Given their buckets of hp, spamming Save or Sucks were the optimal tactic of many spellcasters, but success at doing this, especially early in the encounter, produced a real anticlimax. Alternatively, monster saves against everything which means the spellcasters may have little or no effect on the encounter.
Also, given 3e bosses often had their own Save or Die/Suck powers, PCs could quickly be effectively removed from the encounter themselves.
Given the large amounts of damage both sides were typically handing out fights would often take only a few rounds, which was just as well as PCs would quickly run out of hp in a fight that lasted any length of time.
4e tried to address these issues with special Solo rules for boss monsters, and returning everyone to doing hp damage with most of their attacks, but its a complicated issue that they are still coming to grips with.
I do think most solo monsters need foreshadowing and careful inclusion into the setting. Solo monsters are the subject of legend, and are also the most appropriate monsters for legendary weaknesses.
