When I got the game (on the day of its first release, like all TW games ), the first thing I did was to go into descr_strat to unlock Scotland (Call me a cheater for doing it, but it's my game and if I don't want to play another faction just to be able to play them - screw the developers' intentions!).
Right underneath, I saw the show_date_as_turns line and was like 'WTF?!?' In a (semi-)historic game about the middle ages, I want to see which year I'm in, I don't care about 'turns'! Out with it!

Now. My first campaign, as Scotland, I played on default (2 turns/year). It went ok, but I like playing in a slow-paced way. I don't care about winning as soon as possible. I was appalled to see gunpowder discovered when I didn't even have high-tier conventional units (noble swordsmen, for example) yet.

This is what bugs me most about the standard turn ratio. Time just flies by and by the time you can field significant numbers of high-tier archers to actually use them, they are already outdated and ready to be replaced by gunpowder units. By the time I was ready to build trebuchets, bombards and cannons weren't far-off. Plus: once you can afford a nice crusade (incorporating actual soldiers) the Age of Crusades is almost over - blargh!

Another thing is that you are practically forced to play extremely expansively to be able to aquire the necessary number of held provinces (45-50). There is no time to let the AI become a challenge. As the HRE, for example, you'd never get to see high-tier danish units because they are usually wiped out way before they are developed enough to field them. Same goes for Scotland and England. The constant struggle between the two never happens because one (the player-controlled one) wipes out the other long before other units than militia and the like become available. On 2 turns/year, there is more time for the AI to develop and field more challenging armies than vanillas repetitive town-/spear-/crossbow-militia hordes.

Personally, I settled on a ratio of 1 turn/year. While this slows down the gameplay a lot when compared to vanilla, it doesn't take as long for historic events to happen as with 0.5. I halved population growth and increased upgrade tresholds to deal with too rapidly growing settlements. About 150 turns into my English campaign, I own only ~20 provinces, and I'm happy with it. No faction has been destroyed yet and all are fielding nice armies, not just vanilla's low-tier rabble.

To counter high-tier units becoming avalable too early, I added several triggers to space availability. E.g. Gothic Knights and the like cannot be built before a specific date; Arquebuisers, Musketeers, and high-tech artillery don't become available as soon as gunpowder appears, but require another event about a hundred years later. This way, you actually get to use the low-tier gunpowder units (handgunners, bombards) instead of just skipping ahead to arquebuisers and cannons/culverins.

All this may not reflect the developers' intentions on how to play the game, but so what? I couldn't care less about what purists like foz think about this. It's my game since it crossed the counter in exchange for my money, not the developers', and I can change it in any way I see fit.

To sum it up:
I believe that a ratio of 1 turn/year is the best compromise between vanilla and 'realistic' 2 turns/year. 450 turns is enough to win by playing a slow paced game, while not drawing it out too much (900 turns... )
IMO, M2 vanilla is taking a step in the wrong direction. Epic medieval warfare doesn't mix with fast-paced blitzkrieg - thats what RTS games are for! I would not be playing this game anymore if it weren't for its moddability. Thank you for that CA!