The first thing I fell in love with about Dungeon Crawl Classics RPG (DCC) was the new take on the fighter - thanks to the wonderful 'Mighty Deeds of Arms' mechanic.
The mechanic is granular enough to be intuitive ("You wanna push him over the cliff? You'll need to roll a 4+ on your deed die.") and doubles as a reasonable damage (and attack) bonus. I'd personally prefer a trade-off (e.g. bonus damage or special maneuever) but that's easily house-ruled.
The mechanic constitutes a sub-system of its own and demonstrates that - far from needlessly complicating a set of rules - this approach can provide tailor-made solutions. Using the standard d20 resolution mechanic ("Pushing over a cliff is a DC 15 check.") would have required an extra roll or similar contortions.
The next thing I adore is that magic is inherently dangerous.
This captues the feel of Appendix N fiction very nicely and solves the problem of magic being a reliable everyday resource. I don't want to think about settled wizards casting their daily allotment of spells and thereby competing with craftsmen etc. and requiring me to rethink the whole quasi-medieval world.
Limiting myself to just one more thing to single out for praise, I'll choose Doug Kovacs' outstanding dungeon maps.
I find them very useful at the table. The artistic detail makes it easy to (a) remember what a room was all about -- which means less flipping through the adventure -- and (b) to envision the atmosphere and improvise evocative descriptions on the spot.
Perhaps more importantly, they look so damn cool that they make me want to run the corresponding adventure (or something of my own devising for that map). It's all well and good to have classic blue-and-white maps but to me, nothing screams "Run this adventure!" more loudly than Doug Kovacs' maps.
So what do YOU love about DCC?
(You can find more of my thoughts on DCC at my blog: http://outforblood1.blogspot.de