Many websites when ask you to create a new "username", will limit you in some way, most often like "username should only contain letters and underscores" or something alike.
This results in inability to use my email as username and forces me to come up with new unique string all the time.
What is the rationale behind this?
Why not just let users use their emails, as a simple to remember and always unique?
What is generally wrong with having symbols like *&%$#@ as a part of username?
There are many valid reasons to limit username format.
Uniqueness. If you allow multiple character sets, you can end up with names that look exactly the same, but are technically different. For example, many Cyrillic script characters look exactly the same as Latin characters but are in fact different. "Нarry" and "Harry" aren't the same word, even though they look the same. The "Н" in the first on is Cyrillic, and in the second one it is Latin.
Abuse. It's common to restrict words or phrases that could be considered abusive or offensive to most users. This may include swearing and hate speech.
Readability. If you don't have some sane rules, you may end up with names like "73d37sl29", or "zzzzzzzzzzzzz" which aren't easy to refer to, or compare to other names which could be "73d37s129" or "zzzzzzzzzzzzzz" (note that they slightly different).
Security. You need to prevent the username from itself including code which could compromise the system. The easiest way to do this is to exclude characters that have special meaning in programming languages. Most special characters (e.g. !@#$%^&*()~`;:) have some meaning already, so it's safest to avoid all of them.
Creating a white-list of allowed characters is easier than creating a black-list of all characters that might cause problems, such as code injection due to poor handling of strings.
