My biggest gripe with Ruby is how much syntax is optional and the "accepted" conventions by the community are to leave out a lot of optional syntax.

The worst offender (in my opinion of course) is leaving out of empty parentheses on method calls. It makes it impossible to know if something is a property or a function. The argument is that it doesn't matter if it's a property or a function, but I feel like it makes it very difficult to reason about performance. If I know something is a function, I'll look at the function and see what the performance implications are. Where as if something is a property, I assume it's "free" to access. This doesn't hold when reading Ruby code, and I basically have to look up everything all the time.

Just my opinion as someone that had to switch between python/ruby/perl to maintain tools written during different decades.