The adeptus astartes.

Your army does not have a 'core' that can be targeted. The space marine army is composed of several small, mobile units - all self-sufficient, and capable in their own right. Unit versus unit, your small numbers will see to it that you lose protracted fights, but your mobility and reliability grant you the power to bring several units against the enemy's army at will.

While you can take out imperial guard command squads and platoon commands, thus disrupting the infantry's leadership, as well as limiting their killyness, this is not so with space marines. If your librarian gets destroyed, you've simply lost the librarian. His death isn't game-changing.

An ork army has two large, distinct components: there are the 'boyz' that kill things up close. Supporting the 'boyz' is the 'support.' The 'support' is everything that's not 'boyz' - trukks, trakks, buggies, koptas, lootas, kans, dreads, wagons, kannons... 'Support' is there to make sure the 'boyz' can do the killing. This is achieved by openining transports, keeping units stuck in cover, shooting out elite infantry that the 'boyz' can't handle, and so on. Without support, the boyz are useless. Without boyz, the support won't accomplish much.
If orks lose their lootas, you now control both players' shooting phases. If you lose your three predators, you're down three vehicles. Lots of dakka is gone, yes, but it's still not game-changing.

Tyranids fear the loss of their leaderbugs. It has extremely disruptive effects on their armies, and turns the entity it once was into a gibbering horde. Take out the warriors and tyrants, the army will collapse in on itself.
You lose your sergeants, you're simply down a marine. Some gear's gone, too, and leadership 9, but you still function.

What this is all means is that your army is unique in how it handles itself on the tabletop. Sternguard units, small, mounted assault squads, plasmabacks, predators, land raiders, command squads, bikes, landspeeders, terminator squads, and even tactical squads are all fully functional units in their own right. They don't need other friendlies nearby to accomplish things. It's not necessary to keep them within an arbitrary commandbubble, or create a phalanax so the unit is within X inches of a powerful leader.

Space wolves are dangerous up close, yes. They also want to be within X range of a leader, to gain re-rolls for their leadership, or forgo something in exchange for a permanently higher leadership.
Blood angels are really, really good at killing people, and can soak extreme amounts of damage - but only if they're within X from frail leaders.
Imperial guard require leaders to be nearby, or their weak infantry becomes much weaker.
Necrons cannot function correctly without a leader very close by.
Tyranids demand that you put several commandbubbles on the field. If these bubbles are breached, everything nearby starts behaving erratically.
Tau has a very pronounced core, and build layers around it to keep it alive. Fully breaching the layers, or hurting the core, cripples the army as a whole.

The closest army to yours in composition would be the sisters of battle. Sisters bring a really sharp, really long knife (melta/flamer in massed vehicles), but you bring several (magical bolters, multi-melta, melta/flamer in massed vehicles, mass AP4, mass plasma, constant 3+ invulnerable saves, 2+ armor saves, powerfists, deep strikers, etc.).

Realistically, you'll never have enough points and slots to fit all these tools into a single army, but you can take some of them, or focus on just a few. Options are good, and make your army versatile, rewarding, and interesting to play.

Almost all your units have multiple configurations, and can do more than what's assumed at first glance - the favorite example being mounted sternguard with a handful of combi-meltas and twin heavy flamers.
You can torch infantry just fine, you're the bane of armor, and if required, you can engage in stand-off battles with kraken ammo. Once dismounted, your ride becomes an annoying obstacle, and must be dealt with, or it will end up contesting objectives, plinking off models here and there, and limiting line of sight.

With the orks, you know what to target first. In a proper space marine army, everything is always a threat, nothing can safely be ignored, and no lynchpins exist to be exploited. There are no lootas to shoot first, no buggies, and no obvious super-mega dangerous combat unit to tarpit, or outright avoid, because it happens to be infantry on foot.
What combat units you have ride in reliable transports, on bikes, or inside slabs of armor. When you need them delivered, they will be. Unless you happen to be a stubborn guardsman blob, there really is no such thing as 'tarpitting' assault terminators.

You can spam the table with threats, and each of them must be dealt with and taken seriously. The opponent can't afford to ignore your predators over your rhinos, or your rhinos over your landspeeders.
This is one of many reasons why your book is still so powerful and flexible, despite being the first 5th edition codex.
This is where you draw your strength from.

Use it.
This post officially concludes space marine week. I hope it's been illuminating. Next up, some fun, and a review of the codex as a whole.