Hiya.
Without going too much into detail (I hope! 
)…
HackMaster is one of the most solid rules system’s I’ve seen (and that’s a LOT of systems, btw). I’ve been playing it for almost 2 years now, and I have yet to see a player make a “munchkin” character; tough ass character, sure, but they all have leathal flaws that kill them within a session or two.
The game itself is actually “two games in one”. You can play it “Sanctioned”, or “Unsanctioned”.
–Sanctioned: Basically, you are playing it as a pure parody. You are playing it as the Knights of the Dinner Table play it; style, attitude, the whole thing. A “funny haw haw” style of play.
–Unsanctioned: Basically, you take the humor/parody aspects of the writing style and take it as written: A parody. It’s funny to read, but not play. It’s funny to read a rules system that has sentances like: “Let me say it again. And THIS time I will make it very, VERY clear. There are no contradictions in HackMaster, only ‘appearent’ contradictions or user error.” Would you take the seriously? I don’t. That’s part of the fun of reading HM.
I play Unsanctioned, by the way. As a matter of fact, at last poll, the split of total HM players is 50/50. These polls usually have somewhere around 100 to 500 participents. There are more books sold than HM Association people, however, so Unsanctioned play is probably higher (my guess).
Rules wise, you’re looking at about 18 core classes (plus another 23 from the Spell Slingers Guide; likely another 20+ from each of the futre Fighter, Thief and Cleric book; so another 60+). There are 10 races (some with subraces; like elves…Grey, high, Aquatic, Grunge, etc.).
The rules are based heavily on 1e and 2e, but have significant additions. Honor is one. A characters Honor goes up/down based on his/her role-playing actions, which are based on his chosen class, race and alignment. Charcters who are Dishonorable, suffer a -1/-5% to ALL rolls (or +1/+5% if that would be bad), while Great Honor charcters get +1/+5%. Honor can be “purged” (reduced by 50%) in order to save your ass. There are other uses for it as well, but mainly it’s a role-playing tool.
Critical hits have effects. If you break a bone, it’s broken. You need to rest or get a Cure Critical Wounds (or equivilent magic); a simple Cure Light Wounds won’t cut it. Critical hits require Critical Healing. 
Detailed, but not cumbersome once you get used to it.
Armor degrades with use, but it can be fixed. Armor and Weapons need to be maintained or they start to deteriorate.
A plethora of unique and cool monsters (some damn funny ones too. ), and with the Monster Matirix there are guidelines for manipulation/changing of monsters.
PC’s, NPC’s, Monsters all have a “Hit Point Kicker”. Typically it’s 20 points, but size T creatures only get 10. And ‘nusience creatures’, dont’ get any (like common rats and whatnot). PS: Don’t mess with Dragons! A HM dragon would wipe the floor with a 3e dragon, in a heartbeat.
XP is gained via combat, but a rather large chunk is gained via role-playing your character. I’d guess about 35 - 50% of your XP is gained this way.
Skills (and there are LOTS of them) are % based.
Talents are ‘special ability’ like things. A +1 bonus To Hit with Mace, for example, or maybe Less Sleep.
Characters, when being created, have “BP’s” (Build Points). The player uses these to increase stats, reroll rolls he doesn’t like, buy skills, talents, etc. Characters also have actual backgrounds as part of the creation process. You will discover who your parents were, what they were like, where they lived, if they left/gave anything to you, if you have any family or clan grudges or enemies. When a level is gained, skills might increase, as do stats (each stat has a %'age associated with it; different classes gain different dice to add to that %'age. When the % is 100+, 100 is subtracted from the %, and the stat is increased by one point; ex: A fighter gets d20 increase to Str, but only d4 for Int).
Bar none, the KenzerCo modules are some of the BEST i’ve ever seen! They didn’t simply “convert” the old Keep on the Borderlands module, they actually managed to enhance it. There is so-o much going on behind the scenes…man. If you want a ROLE-playing game, HM is hard to beat.
I’m sure I’m missing a few other things. If you are curious, drop on down to your local Wierd Petes (game store), and take a gander at the book. Or maybe ask to see if a demo game is going to be run (some HMGM’s run demo games at their local stores every now and then). Better still, pick up a copy and read at your leasure. At the very least you will find it a fun and enjoyable read.
If you want anything specific answered, just ask. Or head over to the HM boards (http://www.kenzerco.com/ and look on the left side, bottom for the link to the forums).