I was wondering how long it would take for our Thememaster to jump into this thread. :)
The factor that I consider when determining title/descriptions for the pages is the overall level of optimization/competition for each specific phrase.
If you spend some time at a few engines searching for each of your terms, you'll be able to get a good idea about which phrases you will need to specificly target in your titles, and which ones you can get ranked for using a partial match strategy.
For example, if you do a search at AV for management training and management tips, you will see that both phrases return extremely high page counts. However, the difference between the two is that the management training results has an extremey high percentage of exact matches in the title, and the management tips results do not. In a case liket that, you probably don't need two separate pages to rank well for both terms. (Another great way to get an idea on keyword placement is to use Google's cache. The color highligting makes it very easy to see patterns).
My comments about the navigation structure were really more about link analysis rather than themeing. The two components are very similar, but in this day and age, I think link analysis plays a much bigger role, so my views may differ somewhat from others on the best way to structure content.
I personally don't believe that there are any negative repercussions from cross linking sub topics within the framework of a broad topic. If there is any type of penalty for doing it, it is dramatically outweighed by the added benefit in terms of link analysis.
Using Paynt's pet example, I definitely would link dogs, cats and birds together in the same manner that I outlined in my first post.
A great example of the type of structure I think works best is about.com. Their dog site is located in their House & Home section. Within the dog site you will find links to many indiviual sub directories that address all the various dog sub-topics. You will also find links to the horse, fish and cat sites on every page within the dog site.
That pattern is repeated across all the other individual pet sites, and each pet site links to the House & Home page.
Now from a pure themes perspective one could argue that cross linking exotic fish and horses would dilute their respective themes, but I think it's pretty difficult to come up with any examples that support that idea. Until someone knocks Google of the top of the mountain, I would cross-link sub-topics that tie into your main theme.