I'm not easily impressed. But occasionally, various things will stand out to me in a way that demands a positive response from my brain. This is a list of those things over the past several years of boardgaming. Note that, just because I am impressed with one specific property of a game, it doesn't mean that I like the game, or that I don't have problems with other parts.
Internet forums seem to end up either anarchistic or draconian. While BGG moderation isn't perfect, the system is fairly loose, yet maintains a nice level of maturity.
Through the Ages: Mechanics Mashup
I am amazed at how well this huge jumble of mechanics works together: card drafting, hand management, auction, abstracted combat, semi-unknown future events, resources, development, action points, building, etc. Most games with this much going on feel like uncomfortable overlaps of two or more games.
Race for the Galaxy: Iconography
The icons are so well-thought-out and consistent. I don't see why so many people complain about them. Worlds are circles. Intuitive. Diamond is "the other one": development. Color matches goods type. Intuitive. Card in hand is...card in hand. Eye is look at more cards. Dollar sign is trade/money. Hexes are victory points. That's about 95% of the game. I was so impressed that I developed a player aid for Tribune using this style.
The board looks like a strange version of Monopoly. Each space represents an increasingly modernized type of automobile. The older your factories are, the more losses you take. This mechanism to make you want to leapfrog your opponents. It may have been inspired by the rusting mechanic of 18xx games, but is clearly more forgiving. I don't love every Martin Wallace game, but I am always surprised at how he manages to find something new in a sea of increasingly rehashed mechanics.
Some games have things that may be chosen by players. At any given time, the things have different values based on the game state. Rather than designing the game or the things themselves to be exactly equal/balanced/symmetric, these games make the unselected things more attractive over time. In Puerto Rico, unselected roles get a coin. In Saint Petersburg and Gloria Mundi, cards become cheaper over time. In Vinci (and I assume Small World), unselected civilization combos cost victory points to skip. I love the organic way that this solves the problem. It allows players to value each thing as he or she sees fit at that given moment in the game.
Long ago, in complete ignorance, I had dismissed Shogi as a mere Chess variant with strange symbols. But the fact that captured pieces may be put back into play increases my interest in the game by an order of magnitude. Not only might the game play out in waves (similar to GIPF), but also captured pieces can be stronger than those in play since they may be placed almost anywhere. It would not be strange for one player to have 20 pieces in play, while the other has 10 in play and 10 "in hand".
I enjoy a good deduction game, but the standard abstract ones pretty much all come down to elimination: you collect enough information such that only one answer remains. In deduction games with secret roles (eg Werewolf, Battlestar Galactica: The Board Game, Meuterer), players use human interaction (eg player choices, bluffing, body language) to make deductions and to deceive opponents.
Empire of the Sun: The Map
I like GMT Games maps in general, especially Mark Simonitch's work. This map in particular is rather impressive, clear, functional, and pleasing. Not only does it have to deal with all of the standard type of hex-and-counter issues (cities, terrain, airfields, ports, atolls, roads/rails, labels, resources, borders), but it also has a standard map underneath. Look at all the possible nastiness: tiny islands and juts of water crossing hex sides. It would have been too easy to simply tweak reality to make it conform to the hexes. Instead, Mark shades the "unplayable" land/water areas so that you can see the real untouched underlying map while still having the hex grid be unambiguous. Bravo.
Napoleon's Triumph: Design for Effect
In most wargames that I play, I am thinking about the raw actions that the game system allows, the movement, the attacks, the steps in involved to execute the various phases. In NT, the attack sequence does not feel like a simulation of anything you would see in a real battle, yet the results feel very natural. I find myself thinking at a meta level about the maneuvers and engagements.
Wargames in General
As I quest for more and more meat in my games, wargames are increasingly coming to the rescue. My recent past and present rulebook slush pile consists of Successors, GBoH/SPQR, M&P/NGBG, Gettysburg, No Retreat, Fighting Formations, Downtown, Fading Glory, Flying Colors, For the People, Empire of the Sun, OCS/DAK2, Proud Monster, and D-Day at Omaha Beach. I'm happy with all this, but I'd love to see other genres step up a lot. For over a year now, the newer non-wargames have been dreadful: reprints, expansions, rip-offs, and fluff.