Renaissance is a minor variant of Openquest used in Clockwork and Chivalry 2nd edition. C&C used to be a game set in a fantastical version of the English Civil War that used the Mongoose Runequest 2 / Legend system. Now (in its second edition) it's a game set in a fantastical version of the English Civil War that uses a variation of Openquest for its mechanics. The SRD elides the setting detail, but a lot of it comes through just from having been embodied in the mechanics.
The main differences between Renaissance and stock Openquest are the inclusion of guns and gun rules, including artillery; a passions and beliefs system; a more complex character creation system akin to MRQ2; some minor variations in skills, and two new spell systems (alchemy and witchcraft). The Big Bonus rule is gone, and there are many + / - 10% modifiers. The overall feel from reading it is that it takes MRQ2 characters and uses the Openquest combat system to resolve fights.
I'm not a fan of many of the changes they made, particularly abolishing the Big Bonus rule, which I consider one of the key advantages of the Openquest system. The Big Bonus rule is the idea that rather than offer lots of little fiddly modifiers to constantly change percentages, one should only assign large bonuses or penalties, and only for a small set of reasons (excellent planning and preparation, intrinsic difficulty or simplicity, as a reward for excellent roleplaying). While Openquest doesn't follow this rule with complete consistency, Renaissance does away with it entirely.
As well, the main draw of MRQ2 for me is the combat system. While there are a few other features I really like (how cults are handled, the spell systems), it's the complex, engaging, tactically rich combat system that people talk about. It's the reward for playing the game. To keep the rest of the complexity and strip that piece out feels like they're removing the fun and keeping the slog.
The best part of Renaissance is probably the spell systems, particularly alchemy, which was also a system I liked in the first edition of Clockwork and Chivalry. The spell selection is great, even down to the names of spells ("For to Discover an Enemy", "For the Ignition of the Fires of Passion", "For to Summon Up a Mighty Wind", "For to Bond Together Disparate Things", "For to Fight Like Unto A Wild Beast"). The system itself rewards foresight and careful planning without leaving one totally at the mercy of memorised spells. It's worth downloading the SRD just for this alone.
The equipment list is also fairly strong. I love equipment lists, but the equipment list has been on the wane in many games recently. Many games (even non-d20 ones) just reprint some minor variant of the D&D 3.x OGL equipment. The ones that don't tend to be extremely skimpy, or overly detailed in weapons while ignoring all the various tools a PC might find or wish to purchase. I think that equipment lists are extremely important in games that want to deal with exploration extensively, rather than just combat, because tools and tool use help challenges become real problems with concrete solutions instead of just going "We break down the wall with our swords" or some such. I suspect the main reason that tool lists are so inadequate is because many gamers, like most of the ordinary population, have little experience with manual trades.
Renaissance uses the d20 list as well (technically it uses Openquest's equipment list as a base, but OQ's is one of the aforementioned minor variants of the d20 list) but does a good job on clothing, specialty items, accommodations & eating out, and weapons. It increases the list, rather than reducing it, and it does so in evocative ways.
The passion system is interesting. I'm not a great fan of passion systems (systems that quantify a character's beliefs or factional allegiance), though I do think one makes sense within the context of a game set during the English Civil War. I recall that Stormbringer's passion system was one of the great appeals of the system to people like Akrasia, so if you're looking for a system like that to import into your Openquest games, Renaissance's system is probably what you're looking for.
I think Renaissance will appeal most to people who are looking for a Warhammer Fantasy Roleplay feel but don't like the WFRP rulesets, or who want to play outside of WFRP's Old World setting. It should also be considered by anyone who likes MRQ2 but finds the mechanical complexity of combat overwhelming, or who has a group that does. Finally, people who want different spell systems than stock Openquest offers will find it extremely useful.