They think poker is all about luck, is rigged or a variety of other silly things. They don't see any point in studying the game or improving. Their play is erratic, heavily based around superstition and random impulses. For the most part they just play for fun and will without question lose in the long run.
This group of players (the biggest of the three) take the game somewhat seriously but will ultimately succumb to many of the failings of the fish. Often these are older guys who have played in home games or at the casino for years. They can also be younger guys with some live experience and big dreams. One of the biggest missteps that they will make is not learning the fundamentals properly.
Often they have had success beating their drinking buddies in home games or some ridiculously soft 1/2 game at the casino over a small sample. Like almost all poker players they fall into the trap of illusory superiority. That is, think that they are better than most others or definitely above average at least. This isn't a phenomenon exclusive to poker. 93% of drivers in the U.S. believe that they are in the top 50% of drivers.
They will have a rude awakening online however since the games are a lot tougher than what they are used to. The speed of online play is also a lot faster than live and this means that the bad beats and coolers will come a lot faster as well. Much like the fish they will incorrectly assume that online poker is rigged because of this. Now they have an excuse to never address the actual problems in their game (that they don't actually understand the fundamentals and/or go on monkey tilt the second something doesn't go their way). They can just blame it on bad luck or rigging instead.
This last group is the smallest of the three. Maybe 10% or so of all new players. They are willing to take the game seriously by putting in long hours of consistent play and study. They won't allow excuses to get in their way and will look inward if things are going bad and use that as a reason to get better. Unlike the semi-serious players they do not have a big ego. They are willing to start from the bottom and work their way up the limits. They don't think that they know it all just because they have been around the game for awhile. They often end up succeeding in online poker to a small or large degree on a part time or full time basis. I will discuss a couple of the approaches that they take below.
Learn the Fundamentals
They take the time to learn what a proper TAG strategy actually means in practice even if they think that they already know what it is. This is a huge difference between them and the semi-serious players. Semi-serious players already know everything. Serious players learn (or re-learn) the fundamentals by listening to what top players say in training videos, in books, on forums and in private coaching sessions. And it is not a passive process of learning. They take notes while they watch those videos, read those books etc. Semi-serious players sometimes spend some time on these resources as well but usually it is just to critique the top players or show them their errors. And as for taking notes, they couldn't be bothered with that.
Active in the Poker Community
Serious players join forums and actively participate by posting a lot of hands, seeking advice and forming friendships with other winning players. They listen to everyone's opinion but learn to sort through the quality replies from the noise. They don't get into pointless squabbles about close situations and generally aren't there for idle chit chat either (off topic threads etc.). Cardschat is a great forum to begin with for newer players. It has a very welcoming atmosphere and a lot of people who regularly post good advice. I post there quite a bit myself because of this.
Play A lot
They make hands played goals, not money goals and they stick to them. They learn a lot through trial and error. They consistently play a lot of hands while adhering to proper bankroll management. And they review their big losses as well as their big wins in Pokertracker or HEM on a regular basis. They ask themselves questions like did I play this hand to the best of my ability given the information that I had? If not, how could I have played it differently? They don't tell bad beat stories or lament on outcomes. They simply ask themselves if they played the hand as best as they could. If they feel that they did then they are happy with that and the result is immaterial.
They know that poker is a long term game and they won't allow themselves to make excuses concerning their results. They are only concerned with the process, playing each hand to the best of their ability. They know that they will still tilt from time to time because everybody does to some degree. But they keep it within reason, having the mental fortitude to know when to quit and they never move up stakes to chase losses. They are able to do this because they take the game seriously and know that one bad session or even five bad sessions does not define them as a poker player.
I have always felt that poker is a microcosm of life. The few who are willing to push through the setbacks no matter how many there are, be humble and work hard will succeed in this game. There is no magic formula. All the people out there chasing such a thing will never succeed because it does not exist.