I do usually keep rough outlines as to where I want stories to go. Otherwise I get confused and, as you said, sub plots don't connect. (Which sucks.) Or they turn up in odd places and I have to restructure a huge chunk of the story.

Oddly enough, I think my characters got better after I became more plot driven. I used to look for chances to show off the brilliant creatures I'd created, and bored people in the process. (And often ended up with a lot of WTF moments as characters did something way OOC for them, as that was the only way to keep the plot going.) Doing the opposite - structuring the characters around the plot - actually works so much better for me. Gone are the tedious "let me show you what Emily likes!" scenes. And suddenly it makes perfect sense that Emily needs to collect butterflies for the plot to work out, because from the very beginning, I knew she'd need this ability so built a character who thought insects were awesome or whatever. (OK, made up examples, but still...)

It's kind of funny to me, as it's almost the exact opposite of how I tried to write starting out. I'm not really sure, either, whether it's my method that's better, or whether it's just a method that works better for me. (Probably the later.)