There are two pitfalls that beginning conlangers face:
- Making a language that is too much like English (or another language)
- Making a “kitchen-sink language”, a language which incorporates every linguistic phenomenon you know about
Without exposure to a wide variety of other languages, there is a danger that you’ll end up making a version of English with the words swapped out (a relex, or relexification of English). There’s nothing wrong with this per se, but it’s usually not what conlangers are aiming to do.
However, once you learn about a wide variety of languages, another temptation arises: the temptation to add all the features you’ve ever learned about into a single language. In practice, this makes for an excessively complicated, difficult to learn, and above all, unnatural language.