It has an impact when the person is your liege. Negative opinion means your liege gets less taxes out of your demesne and less troops if (s)he should decide to raise your levies. Other than that, the opinion modifier is mostly used by the AI.
Also, from what I understand, your opinion of your spouse does not impact fertility.
(As an aside, your own opinion is a handy indicator of a vassal being in a faction conspiring against you. Mouse over your opinion of a character and if you see a -10 modifier "Faction member" you know they are up to no good. At this point use standard tricks to get them out of the faction before it becomes too powerful.)
First and Foremost, you should stop playing as a count, it is no longer as straightforward as it used to be. By playing as a weak starting character, you are intentionally handicapping yourself for a game you were completely unprepared for. Try playing as a french duke to begin with. That means that you will have the power of your liege for defensive wars, and can declare war on people much weaker than you, ie rebellious counts and weaker dukes. You should start out only going to war with people who you outnumber by 50 percent, minimum, since you have to deal with seiging and defended bonuses.
You should try not to rely on warfare. Making the right marriage matches and waiting up to an hour for them to come to fruition is part of the game, (playing the long game is part of all paradox games really). In fact, Marriage is actually a much stronger mechanic for taking territory than combat. Not only that, but marriage is the most powerful mechanic in combat as well, because it allows you to call allies with vastly larger militaries than you can command early in the game. An alliance with the king of NearbyNation will let you call in thousands or tens of thousands of additional troops.
Still, the overall point is to stop playing as weak characters. Counts and the occasional 1 province duke in ireland are not advisable start locations for someone unfamiliar with paradox games, because like real life, paradox combat is very swingy. A 20 percent difference in starting numbers can be a 200 percent difference in casualties. That means when starting out, make sure you absolutely have a substantial numerical advantage against whoever you declare war on.
As for your example of losing a battle where you had superior numbers, you probably made a mistake with regards to combat morale, but its difficult to know without screenshots. Even if they had a vastly better general you should not lose a 1000 to 2500 fight, unless you had virtually no morale (probably from not paying the men).
Basic Concepts to Practice:
Revoking a county title as a Duke. This usually starts a war of rebellion that you can crush.
Invading an independent county as a Duke. This includes ones rebelling from their liege that have become independent temporarily
Marrying daughters into a powerful alliance (nearby, but not where you want to expand to)
Marrying sons to inherit land
Keeping your most important vassals and advisors (particularly the spymaster) happy.
Best Answer
It expands the map to introduce more Muslim states in North Africa, although I'm unsure if you need the DLC for that. Paradox also vaguely promises "20 new features", but those might just be relevant to Muslim factions.
The biggest "change" that is relevant for a non-Muslim player, is likely that the AI Muslim leaders need to adhere to decadence. Due to this, their vast empires seem to crumble on a semi-regular basis.
If you don't plan on playing as a Muslim, then I don't think you're missing out on much (but please correct me if I'm wrong). However, the DLC is often on sale for next to nothing either by itself, or in a bundle.