I'm not as concerned as Roy is about streaming in the playoffs, though it can have an effect. I'd suggest a 1-3 transaction limit.
There's a huge difference between streaming in a 20 team league (with 16 player roster), though, versus a 10 or 12 team league (with possibly smaller rosters). Sure, streaming doesn't make a huge difference when you're streaming in Dontae Greene and Jason Collins, but in more shallow leagues, legit guys can be added and dropped.
As for league size, I think 12 GMs is a good number. Having participated in dozens of leagues, 12 seems to be about the right size.
Also, starting positions matter. I like deep leagues. I prefer:
PG, SG, G, SF, PF, F, C, C, Util, Util, Util, Bench, Bench, Bench, Bench, Bench
However, some people like fewer positions, etc.
The other question is categories. There are the standard nine, but then there are questions over whether FTM, FGM, and 3P% should be included. I say no, but it's all preference.