You can learn skills from any trees, but the total number of skills is capped so that you can only be champ in one element, and the more skill trees you spend points on, the less Tier 3 skills you obtain.
sadface.
Are you saying that there's a limit to how powerful we can become? Personally, if I play the game for several months, I'd like to have at least 6 of the 13 skill trees mostly filled out.
I think that you might be disappointed when you see the actual pace that WoE goes forward with. 
Think of WoE as email chess, rather than some fast-paced action game. Because of all the issues related to organizing the event and organizing duels, WoE will move forward at a snails pace. It doesn't necessarily mean it will be boring, but it will be very.. strategical. You will see that "6 of the 13 skill trees mostly filled out" would probably take you multiple years.
Advantage of limiting the skills is that players have to think more carefully which skills they pick, and we won't be in a situation where we have a couple of players with all the skills in the game, running around killing everyone, without any chance of ever getting beaten.
We'll see a steady pattern take hold. Things like Champion of Light and Clockmaker will be the focus of most Light elementals, considering the power behind Stall and Chosen One.
People who work offensively will use Cyborg and Champion of Darkness, with a hand some other areas, because of the level of power that Mage Hunter (still) has.
People who have more of an interest in forced discard will take Predator and Champion of Gravity, because of the devious Equilibrium/Low Point of Gravity/Champion of Gravity mixture.
People who are focusing on quests will take Cloud Rider and Champion of Time, for speed of completing quests.
People who focus on sieging will take Champion of Water and Architect.
etc.
It can be predicted that all players will follow either the above, or another pattern posted on the forum somewhere. It's almost like submitting an FG killer deck.