I don’t think the original game was really that much complex.
Tactically, Enemy Unknown misses the ability to inch forward one square at a time, spending AP instead of full actions. It’s also missing inventory. I don’t miss the first all that much, and I like that EU is more restrictive in what it allows you to bring. You can’t, for example, bring a couple of different rifles. I’d like a limited inventory system, where you could for example pick up a medikit off a downed soldier if you emptied your own inventory slot first, but I don’t think the full Testris inventory would add to the game. In exchange, we have an interesting cover system.
Strategically, EU only lets us build one base. In the original game, though, most of those bases you built were just airstrips for interceptors. They’d end up pretty much where the interceptor zones are in EU - one for each major continent. Eventually you’d end up with 2 major bases, your original, and a second one that was probably focused on research. I do rather miss that it was a good idea to plan base layouts with an eye toward defense. If anything I’d like to see that improved, with more provision for fixed turret defenses and the like.
The biggest strategic change isn’t a simplification. In UFO Defense, you started with the support of all the countries, and you could not do anything to increase your income directly. Your income did increase as time went on, but it wasn’t something under your control. Rather, your investment was primarily in additional scientists, and to a lesser extent engineers. The more labs you had, the more scientists you could hire, the faster your research would go.
The unfortunate thing about Enemy Unknown is that you really can’t afford labs. At least, I’ve never built one on Classic difficulty, because it’s money and power that I need for more satellites. I think I’d like to see a variant where you started with all the countries, with reduced income from each. Satellite coverage then would be about preventing alien attacks, not income, and losing a country would hurt your current income rather than the end-game income. I expect you could probably take a more balanced approach to building that wasn’t so satellite focused.
Positive feedback loops. Games which allow for investment in income almost always have significant positive feedback loops. Absent any controls, it’s archetypical geometric growth issue. The income from your growth allows more reinvestment which creates more growth.
It’s worth looking at the Civilization series for this issue, since the evolution of Civ has been largely about different experiments in curbs on growth. Civ 1 didn’t have any, originally. Civ 3 had a horrible system based on distance from the capital, where each new city had a permanent income penalty, which climbed as high as 90% for distant cities. Civ 4 used a gold upkeep cost per city. Civ 5 uses global unhappiness, with each new city imposing extra unhappiness above the additional population.
UFO Defense didn’t have the problem because you couldn’t invest in income, only research. You could use engineers for income if they weren’t doing anything else, but the return was poor. I’m not sure how I’d change Enemy Unknown while keeping the satellite income mechanic.
I’ve never seen the teleport bug, and I have 350 hours into the game.