Why Are Halide Ions Replaced by Weaker Nucleophiles in Substitution Reactions?

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I'm trying to get a good understanding of why one nucleophile, replaces another nucleophile during a nuclephilic substitution. For instance, halide ions are strong nucleophiles so why are they so easy to replace, even by weak nucleophiles like OH-? I read that a way to estimate the strength of a leaving group, you look at the its pKa when attached to a hydrogen atom. The lower the pKa, the better the leaving group. With the exception of HF, all the hydrohalic acids are very strong which explains why they make good leaving groups. I read that tosylate groups are as good as halide groups for leaving so I'm guessing tosylic acid has a pretty low pKa. So is it mainly the strength of the leaving group that underlies nucleophilic substitutions?

Do nitro groups participate in nucleophilic substitution? Their corresponding acid is nitrous acid which is pretty strong.
 
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In some way, it is all a question of thermodynamics, especially of the mass action law. Typically, the concentration of OH- is high but there are no Cl- ions in the pot, so substitution of Cl by OH is favored entropically.
 
... and OH- is a LOUSY leaving group even after the reaction has proceeded somewhat and chloride anions begin to have a measurable concentration.

In general, the stability of the leaving group affects the rate of reaction with the more stable species undergoing more rapid reaction. Also, the more basic the leaving group is the slower the reaction will proceed.
 
True, but this stability of the leaving group in solution is also a thermodynamic concept.