This is one of the most iconic stories in the bible, so I suspect most of my readers are at least vaguely familiar with it. However, I think there are some interesting observations one could make based on the context of 1 Samuel that might not be immediately apparent.
As we should know, this is not the first conflict between Israel and the Philistines. They have fought multiple battles, under the leadership of both Samuel (1 Samuel 7) and Saul (1 Samuel 14), not to mention earlier conflicts in the book of Judges (e.g. Judges 3:31, Judges 10:7, Judges 13:1). Since the Israelites were previously victorious, this is probably the Philistines attempting to regain control of what was once their vassal.
The battle site is located at Ephes Dammim, which is Hebrew for "boundary of blood". In a literal sense, this probably refers to the battle that is occurring on the site, and more implicitly the "boundary of bloodshed" that existed between Israel and Philistia. In a more figurative sense, the boundary of blood is a reference to the blood of the lamb that preserved the Israelites through the Passover. This might seem like a stretch, but consider the role that David has in bringing deliverance to Israel on this site, not because he is stronger or more skilled than Goliath, but because the LORD brings deliverance through him.
Goliath himself is probably an Anakite, the ferocious giant-men who have been referenced a couple times so far, such as Num 13 when the Israelite spies return to tell the people what they saw in the promised land. This is the first of several Anakites that David will battle throughout his lifetime. It doesn't specifically say that Goliath is an Anakite, but from his size and the weight of his gear we know that this is true.
Out of Jesse’s 8 sons, only the eldest 3 followed Saul to war. Verse 15 tells us that David is still tending his father’s sheep part-time. Jesse has 5 sons who are still at home, but out of all of these he sends David to visit his brothers at the battlefield to see how they are doing and bring some food for them. Sending a young, unarmed boy to an active battlefield is dangerous in the best of circumstances, and while I hesitate to read too much into it, it seems to belie a certain lack of concern about young David. But honestly, Jesse saw David anointed to be the next king of Israel, yet Jesse continues to have David spend his time tending sheep, which at this time in Israel's history was probably a disreputable profession. So through the way that Jesse treats David it's pretty obvious that Jesse does not care much for his son.
So David was held in low regard by his father both before and after he was anointed. His brothers however are now openly jealous of him, with Eliab criticizing David for arriving at the battle site, that David left those “few sheep in the desert” out of his “conceited” and “wicked” heart. Eliab likely suspects that David came to the battlefield to seek his fortune after being declared the next king. Eliab still views David very poorly, doing nothing of significance. Eliab is doubtless angry that David should be honored over himself, and he is insisting that David should return to the poverty of his origins.
More than anything else, what stands out to me about the story so far is David's humility and the pride and jealousy of his father, brothers and Saul. What I see in David is that he raised the sheep in his youth, was given a stunning reversal of fortunes when Samuel came and anointed him as the next king in the presence of his father and brothers, and when his father demanded that he continue raising the sheep, David did so. Even after Saul summoned David to play music and David became one of Saul's retainers, we still see David raising the sheep and serving his father and brothers. David isn't demanding power or trying to gain the kingdom by force, even though he has a divine blessing to take the kingship after Saul.
Meanwhile, we see Saul building monuments to his own glory after being victorious over his enemies. The LORD rejected David's brothers because they have the exact same spirit as Saul. When they see David come to the battlefield, what they in effect are telling him is that the battlefield is a place for great men to change the fate of the nation, and that David does not deserve to be there, because David is only capable of spectating when other men like themselves are fighting. It's prideful because they are insisting that only great men such as themselves should have any power. In the LORD's kingdom, that is exactly the attitude that disqualifies you from having any power at all. Therefore David is anointed to be the next king, and in proper time it will happen.
David insists that he can fight Goliath, not because he is stronger or larger than other men, but because the LORD, who delivered him from other challenges, would also deliver him “from the hand of this Philistine.” This teaches us an important principle: whenever we face a new challenge, remember and dwell on past successes. It is our memory of the lion and the bear that gives us confidence to face the Philistine (metaphorically). Every time I become uncertain about my future or how I will overcome something, I recount to myself what things the LORD has done for me and for others in my life. Three chapters ago, Jonathan insisted that he could defeat the Philistines because “nothing can hinder the LORD from saving, whether by many or by few.” (1 Sam 14:6). In this case David makes a similar argument, insisting that the LORD will save him regardless of the physical circumstances.
Anyway, David insists on using the weapons that he is familiar with, a staff and sling stones (the weapons of a shepherd) and not the heavy armor and sword of a soldier. While David chooses a light armament, keep in mind that slingers could be exceptionally dangerous in ancient warfare, although usually only against massed troops. It’s challenging to hit a single target with a sling: however, being hit by a sling could be fatal. For instance, Judges 20:16 refers to 700 Benjamite slingers “each of whom could sling a stone at a hair and not miss.” While probably an exaggeration, we should understand that slings can be extremely dangerous.
The rest of the battle goes as we should expect by this point. One point that I find moderately amusing is how David is walking around carrying Goliath’s head, as if to remind people of what he had just done. I could imagine some awkward conversation where David sits around eating lunch with his brothers while Goliath’s distorted and bloodless face stared at them. That's kindof how I imagine his conversation with Saul going.
"Hey David, good job killing Goliath."
"By the way, are you really going to just leave that head there while we're talking?"
"Yes, that's what I was planning to do."
Anyway, it seems like one of Goliath's most powerful weapons is intimidation, both from his size and from his verbal threats to the Israelites. The entire army ran away from him in fear, but David ran towards him. David is courageous in the same way that Jonathan is courageous, charging into the enemy because they know that the LORD would give them victory.
Another interesting thing. When David is brought before Saul, Saul does not know who he is. This is surprising because David was already serving Saul whenever the "evil spirit" came to torment him. Not only that, but Saul sent for David specifically on the advice of his servants. It's hard for me to understand how Saul could not recognize him, but that appears to be the case. I'm guessing that Saul just didn't pay any attention to David and just considered him another one of his many servants.
This is the first of David's major exploits after being anointed as king. Like Saul, David starts out on a very good note. Unlike Saul, we can see that David is a deeply humble person who has enormous trust in the LORD to deliver Israel from their enemies. We can already see why the LORD would choose David and call him "a man after his own heart" (1 Samuel 13:14).