If there is one note we often see being given for a logline in the industry at large and in this forum alike; it is CLARITY.
The key to writing a good logline is clarity over all.
You may have the best structured plot with the most interesting timely story about riveting character journeys in your script but unless the reader can understand them all these are pointless.
Of all the meetings I've had with producers and commissioning editors the one element that caught their attention causing their eyes to widen and make them want to know more was the clarity in which a concept was pitched.
"...generally educated but without a specific focused vocation. A bit of a poet, a bit of a scientist, etc." -
Is just not good enough, you need to answer every question a producer would have about your characters and main character in particular in order to write a good script.
In a logline best to specifically define a vocation or if he has none a main pass time to help the reader know who he is and inform the reader that you know who he is. More so if you are yet to start writing the script you will need to do so. Read Lajos Egri he explains the importance of having all the information about a character or as he calls it "the bone structure" of a character.
If he is an observer what vocation could you give him that would necessitate him to be so inclinde? Could he for example work for the very first bureau of statistics established in England? This could be the emergence of the big brother like government observing population agencies and you could use this as an interesting backdrop for the plot. Point is define his position in society and use it to exemplify what type of character he is also best to tie this in to the plot or backdrop some how as appose to it being an add on bit of information.
"You suggest adding a lot of detail here. Isn?t that level of detail a little too much for a logline?" -
It's not the level of detail rather the type of detail that should concern you. Tell the reader what they need to know in order for the specific character whom the story is about in the specific place it is set to be put at maximum danger from the specific antagonistic force before achieving the specific goal.
"The bad-guy is a kingpin, because he controls a diverse criminal empire, just like those other Mafia bosses, drugs, racketeering, prostitution. He is all of the things you said. " -
Be specific and concise, what exactly about his nature will make it especially hard for the MC to achieve his goal, then use that as the antagonist's description.
"Am I wrong? do I really need to spell it out in the logline?" -
Yes and yes. Tell the reader what the MC will do (not blow for blow) so the reader can tell what kind of act 2 to expect. More to the point if you are still writing the script you need to know what type of action to invent for act 2.
Will he use the rise of bureaucracy in 19th century England to have the bad guy arrested? Will he manipulate operatives from with in the criminal organisation to conspire against the bad guy? Will he take up arms and physically fight the bad guys him self?