Combining lineage, class, background, and backstory should give every PC a much broader base of knowledge than I tend to see from casual players.
I'm very much in the school of thought that characters can know anything that they might reasonably have picked up as a result of their prior experiences - most of the time, unless a particular fact or bit of information is something I feel should be either secret or otherwise obscure I don't bother to roll to see if a character might know something as long as the player can give me a decent answer to "How does the character know that?"... (I love it when a player inserts things about their character into the game that tie them to the campaign world.) If the answer seems to be a bit of a stretch, then I might have them roll for it.
But if the character says the legends of their people tell stories about certain monsters, the character probably knows basic facts about them. (Assuming, of course, that the monster in question can be found in the place where the character's people come from...) Rangers, druids and barbarians will usually be aware of the general characteristics of the sorts of creatures that inhabit the areas that they're from, and anyone with proficiency in Arcana/Religion/Nature will probably know some general things about fey, elementals, outsiders or undead depending on their skills.
What they know will depend on how common either the creature or its reputation is.
Soldiers will be generally knowledgeable about unit tactics, basic strategy, and the history of famous battles and the conflicts that their culture has been involved in. A horse archer from a roving horde wouldn't know what a catapult was until they saw one or it was explained to them, but even a grunt in the army of a city-state would know enough about various siege equipment to be able to describe something to an engineer so that they could build one.
If the character's background includes being widely traveled (caravan guard, merchant, traveling entertainer, etc.) and the player states that the character's been to a particular city before, I assume the character has some general knowledge about local geography, customs, government, etc. If they want to establish that they have a personal contact in the area, they can reasonably be expected to be able to gain more detailed knowledge in areas where that contact would be helpful.
In general, when deciding what a character might reasonably know, I break knowledge down into three categories - Personal Experience, Class/Cultural Knowlege, and Legends/Stories...
If the character has personal knowledge of the subject, either through direct experience or study, then they
definitely know common things about it,
probably know some lesser-known things about it, and there's a chance they
might know some deep stuff about it.
When determining whether they know something isn't based on personal experience or study but on things commonly known by a person of their character class or culture, they'll
probably know common things, and there's a chance they
might know more uncommon things.
When the knowledge in question is mainly a matter of having heard some story or legend about something, any character
might know some general things, with a better chance of knowing it if the story/legend in question is somehow relevant to some aspect of the character,
On the subject of planning things in combat, if the players are going to discuss strategy in the middle of a combat round, I give them about
one minute of real time to talk about it, and require that whatever plan they come up with must be something that could realistically be described in a few short sentences... aka the "football huddle".
As a player, I always try to introduce strategic suggestions in combat as something done in character - the barbarian yells, "Vax, bunch 'em up! Korvar,
fireball!", as he shoves the enemy over to the battlemaster who maneuvers them next to the other enemies in the area of effect of the wizard's spell...
