Turns d100 up to eleven. Percentile roll under skill; not hack 'n slash. Decent amount of crunch, but like most Basic Role Playing (Call of Cthulhu, etc.) it front loads in chargen and playing is rules light-ish. Good examples in the text. The Design Mechanism is great at answering any questions, usually quite quickly. The book is nicely done, my biggest quibble is that the font is a little small, except that was done to keep page count down, it contains a lot of content. Gateway to a lot of other high quality Mythras products such as Mythras Constantinople, Monster Island, and A Gift from Shamash.
This game makes combat interesting and realistic, yet simple, with a beautiful innovation that you should check out yourself. The amount of available material also makes it easy to run for those with little time.
Also contains no less than FIVE magic systems and lots of other useful stuff, but see for yourself.
From my point of view it's the best version of the RuneQuest rules yet published. To clarify where it came from, Mythras is the new name for the sixth edition of RuneQuest by the same authors, it was re-named because the RuneQuest licence and brand name was taken back in-house to Chaosium. When RuneQuest 6 was first published it was the first time since the mid-1980s that RuneQuest got a full re-write and I would say it's a more successful version than RQ3 was.
RuneQuest was always known for having a detailed and lethal combat system, Mythras refines this and adds Special Effects - a set of options an attacker or defender can use if they gain an opening, these add another dimension to combat that isn't present in older versions of the rules. Special effects and wound effects tend to encourage different outcomes other than the death of participants. It is much more common that the loser of a combat encounter will end up surrendering (if they're intelligent) or unconscious than a corpse cut up into pieces. It is still possible to take a fatal wound which will kill you in one blow although it's now quite unlikely that a PC will fall victim to a hit like that.
Luck Points - a meta-currency that allows players a few chances (usually between 2 and 4) to re-roll or reverse a roll through a luck point spend. These will drastically improve survival chances for PCs. When previously in RuneQuest you could quite easily lose or get your limb cut off, it's now possible to 'reverse time' and change that critical strike that was won against you, or turn that potentially fatal wound to your head into 'just' a serious wound.
Passions - adapted from Pendragon which add a flexible way of expressing the most important aspects of your PC - what your PC hates, loves, drives towards, and so on. These can be tested to make a difficult decision, test conflicting situations and occasionally buff certain skills.
Toolkit - the rules are presented as a toolkit - there's a lot of detail and it's not necessarily intended to all be used. The core rules detail five magic systems, it would be very unusual for all five systems to be used in a single setting, the Mythic Britain campaign supplement only uses Theism (Christianity) and Animism (Druids) for example. You could easily run a campaign using only one of the magic systems or none depending on your setting and a high fantasy setting could use all.
It's not designed for beginner GMs but can easily be run by a GM who knows what they are doing and a beginner group. New players can hit the rules like a vertical cliff so I think there's a careful job to be done by the GM not to overwhelm players and be very selective about which parts of the rules they're using. Special effects can produce an analysis-paralysis in play because of the number of options. I would recommend the GM pre-empting this by making the players choose a few special effects that their PC knows out of character creation, or make a selection for each player to begin with and allow them to expand what they use further into the game.
The toolkit nature of the rules can be confusing I have noticed there's a tendency to want to take on all of the rules all at once from the get-go. This is probably a mistake unless your game group is very keen on sticking to all of the rules all of the time. You can simplify a lot at the start and ramp up the detail as you go, as long as everyone knows this is what you are doing from the start. It's not really a problem with the rules but more a consequence of how comprehensive the core rulebook is, you can comfortably run a campaign with only the core rules.