Many German words are similar (or even the same) as their English counterparts so it's relatively easy for an English speaker to learn German vocabulary in comparison to, say, Hungarian.
It's the little pieces of grammar that are confusing.
Zum Beispiel, die Schule, in der Schule
Wir gehen ins Kino, Wir gehen in die Kneipe.
der Schlussel, Ich kann den Schlussel nicht finden.
der Tisch, die Flasche steht auf dem Tisch
I'm not too clued up about dative and accusative cases, I tend to learn things mechanically eg after the word "auf" you say dem for male/neuter words and der for female words. You say zur for female words (zur Post) and zum for male/neuter (zum Hotel).
When learning a foreign language you learn things about your own language that you took for granted previously.
For example, the word "when" in English can refer to past or future.
When I was a child.
When I leave school.
In German, you have two separate terms - Als and Wenn (and Wann for expressions of time).
It's the same with the word "could". This covers two tenses, in German you need konnten or koennten.
Wuerden and Wurden are also confusing, took me ages to get the hang of that.
The number of words for "you" is also confusing for beginners. The word Ihr/Ihre is also difficult because it means so many things.
Spelling I found easy but plurals often involve some guesswork.
I still get confused with mich/mir, dich/dir, sie/ihr etc. (does this happen to Germans too?)
Seit is also strange for English speakers. Ich wohne seit drei Jahre hier. Here we are using present tense but in English it would be past tense, I have been living here for three years. Which is why many Germans say things like I am working in England since two years, when they speak English.
The German future tense is different in usage. In English if you ask somebody to do something for you right now, the might reply "I'll try" which is future, but something they will do right away. In German you stay in the present tense here, Ich versuche, and only use Ich werde..... for events some distance in the future.