Pronunciation is definitely the biggest thing that people notice when you are speaking English. Let me tell you a personal anecdote about this.

Michal and I once went to a conversation class taught by Americans. Before the class started, the teacher said to us: “So, do you speak good English?”. I replied “We think so”, and the guy said “It sure sounds like you do”.

Notice that I just said three words, and the teacher could already tell if my English was good or bad. Why did he think my English was good? It wasn’t because I used difficult words or advanced grammar structures. It was the way I pronounced English words.

When you talk to a person in real life, they may not notice your limited vocabulary or grammar mistakes. But they will notice right away if your pronunciation is good or bad. If your pronunciation is poor, they will think about you as the guy/girl who speaks bad English, and good grammar and vocabulary won’t help you!