While I was writing this up keithLamothe and Asharak both posted elements included below. I'm going to post it unedited anyway, though, as it's different enough that I think it's worth it.
The focus system appeals to me a lot. I've never played a game with it before but it sounds like a great idea. However, i think that, if implemented, it should be more than just the option to focus on a one or two technology trees; you should have more control than that.
From both realism (even though realism isn't that important to me) and gameplay standpoints, focusing on, say, agriculture doesn't make much sense. agriculture is really broad, and can vary from plowing to cultivate more land or irrigation to cultivate undesirable land. As many people have said, new advances are usually based on need, without a clear picture about what the solution would entail. If my empire is on lush terrain crisscrossed by streams and rivers, irrigation isn't going to help much. On the other hand, the plow would allow my empire to cultivate vast tracts of these fertile lands. Or an even more drastic comparison: the cotton gin vs. the plow: if I have a lot of land, but little of it is suited to grow cotton, then the cotton gin is going to be a significantly inferior advancement to the plow.
You could always say that if the tech tree is designed and balanced well, then all technologies will be helpful. But that is inherently wrong unless the game is always the same. All advancements should have a reasonable opportunity to be worth their cost when compared to others, but they shouldn't all always be worth just as much to the player. Every game should play differently, due to starting area, strategy, events, etc. Certain circumstances present a strong impetus for certain advances - both in real life and in a game like this.
Given that Stardock has plans to allow for truly long and epic games, I am hoping that they are also planning a very, very large tech tree so that by the end of the longer games not everyone has everything. There's the option to make technologies take longer on larger maps, but I personally hate that. It makes me feel stagnant.
If we take a large, detailed tech tree for granted (for now), then my proposal is literally a direct compromise between focused and specialized advancement. Have the tech tree divided into two levels of categories: the broad top level (agriculture, industry, military, etc.), and the bottom, specific level (swords, farmland improvement, farming tools, armor-piercing weapons, metalworking, etc.). This should be ONE big tech tree, and the specific categorizations should draw from multiple broad ones! Then, let the player choose to focus on ANY of the categories from EITHER categorization level.
That way, if you loooove swords you can focus on swords. Or if your enemy has really tough armor, you can focus on weapons to negate their defense. Or, if you're at peace and want to improve your agriculture, but you don't really have a vision for anything specific, you can focus on agriculture as a whole and be surprised!
I think that in the end, a system like this would provide the replayability granted by the SMAC system, but also provide the player with reasonably specific control over what they're going to get. The specialized categories would be organized so that whichever tech the player ends up researching will be just as good as the other potential advancements s/he could've gotten. No more feeling of being swindled by chance
The rest of this is a little unrelated/going into details but I'm going
to post it here anyway.
Some other features I think would be nice:
1) Techs should be able to be part of multiple categories if applicable.
2) Techs should be able to have prereqs outside their categories. This is already implicit in the idea, because it's based on one big tree, but categorized separately into related technologies. They should also be able to have material prereqs - no swords research if you have no metal.
3) Some specialized categorizations shouldn't be available until certain prerequisites are satisfied. You shouldn't be able to focus on swords on the first turn if you have neither metalworking nor any metal resources, nor even the concept of a sharp-edged weapon. Maybe draw from the galciv approach here - before you unlock the "swords" category you need to research the "sharp-edged weapons" tech, as well as have metal resources. The "sharp-edged weapons" tech could be part of the "weapon concepts" and "armor-piercing weapons" categories... "Axes" could also be unlocked by researching the "sharp-edged weapons" tech, but it could require stone instead of metal. Or either!
Note that this are just examples, and not particularly good ones.
Some problems to resolve:
1). What happens if you want to focus on a category with no available techs (haven't researched the prereqs)?
I can think of two solutions: automatically research prerequisites or don't allow you to do this. I'm leaning towards automatically research prerequisites. If more than one prereq is needed, maybe make a high chance to consecutively research prereqs for the same tech until it's available. If you can focus on multiple categories at once, maybe it will prefer prereqs from other categories you're focusing on. Could be used to improve chances of getting what you want.
2). If there is too much interconnection between the specialized categories then the focus system would fall apart - it would take ages to get what you want. Would probably end up slowly creeping up many categories simultaneously.
The only real solution to this is to design the tech tree very cleverly, but I think it can be done, and done well. And if anybody can, it's Stardock!