I'm thinking about buying Dawnguard soon, but I have some questions about it. Is there any new content apart from the added storyline Dawnguard? I know there are some new items like crossbows and maybe (armor)? Are there many new items? And are there things you can do/find/kill/get in the normal Skyrim landscape? Sorry for asking so much!

There are new armors, new weapons (including Dragonbone), new spells, new powers, new shouts, new creatures, new NPCs, new missions, new areas, new storylines that tie into DG + vanilla Skyrim, and so on.

Monsters include new variations of old enemies (vampires, Falmer, dragons, chaurus) and some brand new ones: gargoyles, death hounds. All of them can be encountered in vanilla locations. Also, there are wild werewolves (but they can't infect you with lycanthropy). And some enemies you only meet in the Soul Cairn, but that's connected to the storyline.
Dragon bone weapons, lots of faction armor, two kinds of Falmer armor.
Useful new artifacts (aetherial stuff for everyone and unique vampiric artifacts that enhance vampiric and werewolf powers)
New alchemy ingredients.

Werewolf and vampire perk trees!

Dawnguard is not somewhere you go outside of Skyrim like Shivering Isles was to Oblivion for example. While there are some vast new areas, and even a daedric realm, all the action takes place within Skyrim. All the powers, armor, weapons, skills, etc you get are usable in the normal skyrim landscape. There are also two new perk trees for vampires and werewolves.

Not *entirely* true. Shivering Isles, yes, was a new and different map accessed through a portal. There are some AREAS that exist like this, that are entirely new (creatures, textures, flora and fauna). However, they do not exist on their own (separate) maps. IOW, you're still in the same sandbox, but there are many new areas.

I never bothered with smithing much then when the Dragonbone weapons was added it cost me thousands of gold something like 20000 or so. Cool weapons the Dragonbone and the arrows.

Although simple, smithing arrows is very useful indeed, especially if you're on a console.

Well, there's new areas, armor, weapons, clothing, jewelry, stuff of that sort. The actual questline and a few other side quests, new enemies, and variations on old enemies (including Revered and Legendary Dragons), and werewolf encounters at night.

It adds MANY things.
Several new quest lines.
Several stories that are profound.
Beautiful scenery and panoramas.
Plenty of new items.

TONS to explore.

The falmer become significantly more interesting.

AND: you get to kill A TON of those verminous bloodsvcker vamps.

Everybody !
Don't forget about the boat service in the port cities !

It add's alot, and dont let somone tell you other wise.

Thanks everyone for their answers! Im going to buy it soon enough!

About the storyline, does it also play in the normal land. Or am I totally wrong and is castle dawnguard in the normal skyrim landscape? At last i'd like to know if it's easy to miss stuff? Like new dungeons or monsters. I hate to not know/have everything haha

The Dawnguard Castle is technically in a different worldspace, but it's part of Skyrim (just so far away off coast).

Castle Volkihar - In Skyrim, off the north coast.

Fort Dawnguard - Inside a hollowed out, open topped mountain kinda, near Riften.

New weapons, armor.
New types of Dragons.
Castle Volkihar, Fort Dawngaurd, and Soul Cairn.
Wild Werewolves.
Vampire attacks.

New dragon types. New vanilla monster variants along with a couple of new ones. Arrow crafting, Dragonbone weapons, and some new armor types. Those are some things that aren't related to the main story. Oh and boat travel and wild werewolves, but they are quite rare so.