The BR conversion isn't the smoothest conversion out there. And sadly, Wizards will probably never get around to a full 3.0/3.5 since those pesky Planescape fanatics seem to be cheering a bit louder for an official PS conversion
The Domain Turn rules in the new conversion are off and I still don't like the mass combat system (of course, I never liked the old mass combat system so I guess this might not be a fault of the conversion process).
I played in two unfortunately shortened Birthright campaigns, one 2e and one 3.5. Neither of these campaigns really matched up with the more strategic ways that I think BR was intended. Instead, they were definately more tactical and focused in the sense that the PCs were all working with each other and weren't high-blooded nobles.
The first campaign had an excellent group with it and an excellent DM. We started out at a tournie (and at first level) in Brecht where a noble girl was kidnapped. We, as the heroes, immediately gave chance and found herself up against some of what appeared to be the Gorgon's lackies. Via prophecies and need, we ended up in some of the ancient forests of the elven lands and came across a cursed elven city. We managed to break the curse which freed the hundreds of trapped individuals there for the first time in centuries. Hence, a bunch of low-level commoners all looked to a few people for help...us. Suddenly, a bunch of 3rd and 4th level heroes were having to lead. It was great. The characters each had their own areas of specialty, with mine setting up a temple to the goddess of learning and having to manage to clear out the catacombs they discovered while trying to deal with his feelings with two PC ladies with the Gorgon turning his gaze upon them from far away (or at least allegedly from what we could tell). The warrior PCs were busy trying to set up defenses for the city (we were actually only in part of a giant, ancient city. We had reason to believe a liche was based in the catacombs that ran beneath the city with a dragon about ten miles away just outside the other end of the city and I believe there was one other major power player there). We also had correspondences going out to the elves and dwarves. We were trying to work frantically at both a local and national level, but were definately focused more on the local level things and our own personal things.
That paragraph hardly does it justice but it was great. Birthright demands a highly political feel to it. I don't think it is so much the rules you or any DM needs to play a good Birthright game, but rather just the feel setting of the setting. If you might want a good novel/series that really shows how a BR setting should be run, I'd suggest George R.R. Martin's
Song of Ice and Fire. It shows how politics rules the world and how people who aren't actually evil can still be "the bad guys".
The second campaign was ran with a group of heroes as the work horses for a lord of a fair-sized city. The heroes were his lackies and doing what needed to be done for him...investigating this, looking into that, making sure peasant riots in another city didn't spread to their own, etc. This was another great campaign because the PCs integrated themselves into the setting and each one had more then just what his Lord wanted him to do. One PC was hitting on the lord's youngest daughter while another was looking for his lost brother and another was trying deal with his rising power in the church.
I think a real BR setting should also make the characters feel like they are, at least at low levels (which I also think every BR campaign should start at), part of a great machine. They will have people commanding them and, while they might not like that, they'll generally have to to what their told. Plus, as they grow in level and power, they have to also grow in responsibilities. That "down time" won't be lounging around or just training - guards will have boring patrols to full-fill, new guards to train, and supplies to be counted/catalouged.
At low level, make the PCs feel like the rank and file. They are special and their higher-ups know this, but one level of fighter isn't that much different from a level of warrior. They'll still have to earn their respect and power.
Also, start out in a small local setting. Flesh out the immediate area of the PCs lord/boss and his vassals/underlings. Things happening at a larger scale are mostly just rumors and conjecture and likely something the PCs won't have any real power in. Their initial campaign focus should be on why a five farmhouses were found ransacked and burnt to the ground. The world-spanning campaigns will naturally build from there. Let the PCs world slowly expand along with their power.
I think this is probably the most important piece of advice for BR - everything is political! Those farm houses are because the Gorgon is sending in hired human mercenaries posing as raiders from a neighboring kingdom to sow unrest between the two kingdoms. The cleric didn't just happen to get sent to a neighboring city to show his Church's respects; he's there to recieve a piece of evidence that the King's uncle is still alive and looking for allies to overthrow the King/Userper's throne (too bad no one told the PC about the highly treasonous secret message he'll be carrying back). The PC who was trying to marry the Lord's daughter was probably less then honorable, though a good man, and while his intentions were not entirely political or selfish (he was a good natured, though chaotic, bard of whimsical nature) he was definately delving into a hotbed of politics (without really carrying or realizing the possible consequences...oh well, his problem).
Anyway, thats the major points of a BR setting. One sticking point we found in the second group was, after the initial introduction adventure we were a little mixed on if we wanted to be a "free-roaming" game where we, as PCs could break apart and carry out our own agenda (as originally planned) or if we wanted to stick together as a group and progress in a more traditional adventuring group. That might be something your group might wish to debate about if you go with the same approach we did.
Anyway, good luck if you decide to go through with the campaign! It is a great and wonderful setting and has tons of potential. Hopefully some of my advice might help. Sorry if its aimed at lower levels of play, but thats what I have experience with in BR. I still loved it though and hopefully you will too.
*chalks up another hopeful BR fan to cheer for a true BR conversion*
