Viability of tech trees other than combat
Two points:
1) Beelining to a technology doesn't seem to work. That may be by design, but the increases in cost from one tier of techs within a tree and the next tier prohibit doing much in any tech tree without improving your research capability significantly, which requires that I go up the civ tree to get those bonuses.
2) Force multipliers based on multiple figures in a unit means that I'm all but obligated to go up the combat tree to get to squads and such.
Now, if I want to specialize in magic, I can either take a long time to research up to staves at 4 attack each, put together mages after getting up to squads in the combat tree, and then have them be decimated in combat from anyone who's focused on the combat tree and put together equivalent tiered units from there with axes and armor and such. To make the other trees viable, equivalent combat power needs to be placed within the magic tree at roughly the same research tier as in the other trees. Equivalently, there should be defense/dodge enhancement equivalents within the magic tree to armors in the combat tree. Civs, to my mind, is the tree that makes it so you don't HAVE to have combat; you should be able to buy off armies that come into your borders with influence, making them flip, or to influence the monsters in your territory to go fight on your behalf, making sure your hands stay clean. That may exist, but I've not been able to make it work; I always get told that I can't recruit the ogres/trolls/whomever in my ZOC. Freeloading bastards.
Alternatively, and this might not be possible within the current game engine, but it might be nice to have research advancing equivalencies in each tree. Civ gets civ research, which applies to the civ tree, combat gets combat training, which applies to the combat tree, and magic gets magical research, which applies to the magic tree. That way, you encourage specialization to a specific tree, rather than make it so I have to jump around through different trees to put together a viable strategy.