I've always stuck by my theory that in order to consistently win yahoo H2H leagues, you need 2 things: specialization and depth. Positional diversity should always come after these 2.
Specialization - You always want to go into a draft with a strategy in mind.
a) If I get the top 3 pick, who am I going to draft and what kind of team should I build?
b) If I get a late round pick, who am I going to draft and what kind of team should I build?
c) I don't care which pick I get, I'm going to draft a best possible big-men dominant team
d) I don't care which pick I get, I'm going to draft the best possible small-ball team
Each player you draft should strengthen your core categories (6 or 7). Try not to draft players that contradict your strength categories, i.e. drafting D12 for a big-men specialized team and then drafting a Corey Maggette to strengthen FT's doesn't make any sense. Instead, you rather take another poor FT shooting big like Griffin to augment the categories you are already strong in.
I think spreading your team too thin will always result in you being mediocre across the board. On the other hand, if you try to specialize in 6 or 7 categories (while punting 2), you will be much more focused in your strengths and more consistent.
Depth - You rather have a team full of above average players who are consistently producing rather than a team with 3 mega-stars and a plethora of waiver wire garbage. So if I have a stud like Varejao who was going ape-s*** crazy in the beginning of the season, I would much rather trade him at his peak value for a Hickson + Asik because it provides me more depth. My FG% and rebounds will be much more stable with 2 double-double bigs than simply having Varejao alone.
Lastly, positional diversity comes into play. I tend to just stick with the general rule of thumb that you want at least 2 players of each position. I've won leagues with having only 2 PG's or 2 SG's or even a single PG at times depending on how specialized you are.
For example, if you're building a big-man team, having 2 PG's, 2 SG's and 2 SF's are probably enough while the rest of your roster can be PF/C's. I found this to have much less impact to the overall success of your team compared to the importance of the first 2 concepts mentioned. The reason being is that aside from Wednesdays and Fridays (sometimes Saturday), you're never going to have a full roster going. So it really doesn't matter all that much how much positional diversity you have. You just want to maximize the best talent possible for the positions that you DO have in order to stay competitive.
*rocking the new blamer sig*