Try to have a good base within most areas, esp. the practical ones if you're going to do real penetration testing and not just audits where you have a checklist and full access to the computers.
With that base, specialize in one topic, not something as small as "RFID" only, but rather Exploit Development (programs), Web App Sec (websites), Reverse Engineering (e.g., RFID but also other things), Malware Research (this includes reverse engineering, at least at a basic to preferably intermediate level), Social Engineering ("human hacking"), Wireless Security, Mobile Security, and so forth.
You can also specialize in the more theoretical part that includes reading tons of RFC's about e.g., how protocols, like TLS and SSL works along with all the other protocols, so you can e.g., instantly spot if a protocol has been incorrectly implemented in embedded devices such as mobile phones. (Just an idea.)
It's impossible to know everything for starters, and we all have our blind spots as you should also know about technology (e.g., server architecture) in general, meaning that having a good base around computers, is a good idea.
If you're really good, then you can specialize in just one topic, but then you have to focus on that and only take the basics of the other areas in to aid you with your specialization and e.g., expand upon that. Being really good means you know pretty much everything there is to know about a topic on at least a basic level, and then have a lot of knowledge on intermediate level, but also some expert knowledge that perhaps you've discovered yourself through research, which often takes from a week to several months (or longer).
Becoming the "basic hacker" that just runs tools, is something almost anyone can become nowadays with so many resources available, videos, very good books, a huge set of tools that hasn't always existed (except nmap?
), etc. Two teens in my home country recently discovered a gaping security hole in a program used for printing or system info, within this program was the master password to the entire infrastructure that control the entire school system, and these two teens are 14 years old.
Often I consider myself as a noob when I compare myself with those that almost speaks gibberish to me, because it's on such a high level that I have trouble keeping up, one area where I'm honestly somewhat blank is kernel overflows in Windows, and diffing binaries such as Windows patches, where an introduced part of assembly fixes the problem, but the complete meaning is hard to get.
Another would be reading the entire WiFi IEEE specs, and e.g., discover Hole 196, it's stuff like this that is either: "How the.. Did they discover this?" or "I wish I knew what all this meant" (and sometimes I sit for hours reading about various topics in e.g., a blog post, trying to understand the inner workings of certain topics that's outside my area of expertise.)
It's just my 2 cents for the day