In the past year, I've gotten much better at Dominion. The main improvements I've made in my game have been looking at my decisions -- those "universal truths" I thought I knew about Dominion, and questioning them all. Trying to see if I could do better. And I have over 10 iso levels worth of improvement to show for it.
Most Dominion players under-value trashing. Except at the highest levels of the game, it's far more common for a player to focus too little on trashing than to focus too much on trashing. Consequently, most Dominion players undervalue Urchin and Mercenary; they buy too few early Urchins/other attacks, and thus on average they don’t get their first (or the usually desirable second) Mercenary early enough.
For example, in this game
http://www.gokosalvager.com/static/logprettifier.html?20150403/log.516ce511e4b082c74d7a526a.1428083504568.txt I have bad luck early: despite opening quadruple Urchin, I don’t play my first Mercenary until turn 9. But my opponent only buys two Urchins on turns 2 and 3, plays an unfocused strategy, and does not gain Mercenary until late, so I am able to come back and build an Advisor-Festival-Peddler-Courtyard engine to win.
Most Dominion players would get better with Urchin if they started from the premise that you should prioritize getting two Mercenaries if you are getting it for the trashing. Colliding your Urchins early gives you a large advantage, and some people don’t like that, but knowing to prioritize getting two Mercenaries on many boards sets you well ahead of other people strategically, which makes up for a lot of bad luck.
Edit: Unless you are a really great Dominion player, if you are tempted to skip on Urchin #3 after already colliding, you should be aware that Urchin #3 is generally better than Silver here, and most Dominion players incorrectly buy Silver way too often in this situation. A typical level 20 player who changes to always buy Urchin here will have improved their Urchin play substantially.