To add on to everyone's comments, particles are also usually dropped when speaking casually. It's very odd if you use them all the time.

@Ann

N2とN1の文法はほとんど論文的な文法なので普通には使わないでしょ？会話するときに簡単のを使った方がいいと思います。

Japanese's grammar is too difficult to use when your level up.

Ｎ２とＮ１の文法ではたくさん文法がかなり同じ意味だが、状況によって、文法の使い方はちょっと違いますと思います。

そして、話す前に、いつも　考えて、頭の中で　”どんな文法を使ったらいいですか”　と聞いていますから。。。

They often use "I", "you", "he/she" far too much. These words are often omitted in Japanese as it's usually clear who is being talked about.

Pronunciation. 

Placement of stress. 

Worrying too much about the various conjugations of verbs. 

I believe when some foreigners say things like こんにちは, they don't really give the ん it's "full value" I guess? They pronounce it more like こにちは rather than こんにちは. It's not that major, but I know I used to say it like that when I started out :p