Getting into some things that I found of particular interest:
I mentioned that some classes and races were missing. Illusionists and Barbarians. While Barbarians didn't come into 1e until Unearthed Arcana, if I recall correctly they were available in an issue of The Strategic Review (which later became The Dragon and then Dragon magazine) as were Illusionists.
Missing races include Gnomes and Half-Orcs.
Races and Classes available in a campaign has always been a matter of personal preference for a DM and their campaign world. As a note, my current preferred system Labyrinth Lord (Advanced Edition Companion) is also missing Barbarians.
The sleep spell is a fun convergence point among the retro clones including Swords & Wizardry, and a source of house rule discussions routinely. I do not recall a retroclone version of the sleep spell that has the area of effect clearly defined, instead relying on the number of hit dice effected to handle that matter. For a rules lawyer that could conceivably prove very interesting . . .
Example: "My 5th level wizard casts Sleep at maximum range of 240 yards outdoors into a field. There are no visible enemies and if there are not an enormous number of field mice, small birds, insects, etc in that field then the area spreads out until some people in the next town over suddenly and inexplicably nod off for a nap."
Retro clones are full of little things like this, as are the original rule sets they are based one. That is part of the charm and Swords & Wizardry Complete suffers no drawbacks from any of it.
In my overall skim-through of the rules I found nothing that really stuck out at me as a negative. S&W lends itself well to the style of play it is designed for and contains plenty of optional or alternative methods of running games or handling rules.
You should take a little time and go to www.d20swsrd.com and look it over to see if some part of it might be of use in your campaign.