Starting out of the block, the best player in college can have a good chance playing against anyone ranked #100 and above, just by making less mistakes, playing consistently and without having alot of weapons, which is the case with most college players.
Getting points from ITF's and having to qualify for WTA events is a tough and difficult road and I see many players have some success one year but aren't able to defend those points the following year and drop back in the rankings. To get those points at the ITF level (100k and below) one has to be quite successful, which is like the feeding ground for everybody else.
My point is that the toughest component is a mental one, that of not winning like one is accustomed to in college. So much about tennis requires confidence, so that is the biggest quality, which gave Mal the extra boost she needed to even decide to turn pro.
Second component is one has to have strong ground strokes to be able to hit winners and more importantly to control points. In college, players that need 3 more balls before they can hit winners won't make a living at the pro level. One has to be able to turn it up without hesitation. I've seen Mal do that successfully this year against Serena, something she couldn't do against Sharapova last year. In addition, she was hitting deeper balls than before, not allowing Serena to rip winners more frequently.
I think the third component is the serve. At the pro level, one has to hold their own serve or lose. In college, one can get away with trading breaks, but would have a more difficult time having success at the pro level. Mal's first serve has improved and saw some aces against Serena, but her second serve is target practice and she will need to improve that to get to the top 20.
The last component, I would say is being able to neutralize an opponents power and counter punch with one's own power for winners. I see Azarenka do this so well, something that Sharapova hasn't been able to do against Serena. At a lower level, being able to retrieve most of the balls back and wait for the opponent to make errors, may win games against the less consistent players but won't work well against the top players.
Gibbsy has a mentally strong game and she's very intelligent enough to work her way back into a match. Against weaker opponents, she'll make the top 100, maybe top 80, as of today. She'll need to transform her game like Mal has, in order to make top 50. She's not that big, so she doesn't have the power that Mal has and McHale was able to climb as high as #24 without any real weapons. However to maintain that ranking isn't so easy especially being consistent for so long, players will know to just get aggressive and take those points for the win.
When you look at Madison, she's got 3 components, power, aggressiveness, and strategy to out-think her opponents. She isn't as consistent yet but that may occur over time. Sloan has speed, counter punch power, and consistency, which has propelled her to the top 20's, but has yet been able to out-think her opponents. You combine Sloan and Madison and you have a top 10 player.