Certainly on Classic difficulty the first major hump is the first Council Mission. Wipe and you're going to have to restart. Refuse it and you're locked into strategic fire fighting to stop losing countries, and may not be able to go for engineers on Abduction missions. Play and abort can be worth it if you get a promotion or two before you leave. Win it and you get a pocket of breathing space.
I also think that you need to choose right from the beginning whether to focus on weapons or armour. If you try for both you're constantly behind the curve and every battle is too close for comfort. It comes down to personal preference which you choose, but also which classes you have. Sniper and heavy like weapons, assault and support like armour. Personally I beeline laser weapons and go for the Sergeant Zim approach of killing them before they kill you. Whichever you choose, by the crunch point of the alien base (or latest the overseer) you should have enough scientists and alien autopsy bonuses to catch up on the unexplored research paths quite quickly.
One more reason for choosing weapons - you can never recover alien armour, which makes it more expensive. Plus the light plasma rifle with a SCOPE helps you carve out a creamy little early-middle game pocket where you float like a butterfly and sting like a venomous rhinocerous. Course, your unarmoured rookies with arc throwers might have a different opinions, but they're just grunts, they can't possibly make this kind of decision. No offence
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