Something to keep in mind about classical composers is that keyboards have really only been tuned in equal temperament for maybe 150 years; equal temperament is a compromise tuning where all the intervals are exactly alike, but none of them are *perfectly* in tune (5ths are closer to perfect than 3rds are). If you tune the intervals to be better in one key, all the relationships will change for other keys, so each key really will have a different character. And that's the situation that baroque and early classical composers were writing in ...
They still used tempered intervals, often trying to work it out to give themselves as much flexibility to write in different keys as they could (a lot of the structure and form of clasical music is based on changing from one key to another and another and then back again) but the techniques for tuning a piano in precisely equal temperament weren't worked out for a while. That's just keyboards, of course -- voices, fiddles, etc. can tune their intervals as they like unless they're playing with other instruments that can't.
Does that help any?