I've played Terraforming Mars twice and agree with Watno that both times I could not figure out what it is that people like about it. I don't think it's the worst game ever but I definitely don't plan on ever playing it a third time. There are a lot of things it does badly and/or has made me realize I don't like:
1. I don't like drawing cards from a deck (I also dislike RFTG for this reason, and reason #2 below). Dominion is somewhat of an exception, but I think it's a little different because you're the one setting up the deck you draw from. I don't automatically dislike games with cards in them - any kind of drafting or otherwise self-balancing mechanic makes cards fine - but there's something unsatisfying about just winding up with random cards in your hand that you had no control over. Arguably the drafting variant helps address this, but with such a small hand of cards you're drafting from it doesn't help that much (but it does slow the game down even more, exacerbating #3 below).
2. I don't like games with a million different cards that all do different things. This just makes it impossible to plan for things, learn the game, and internalize what you're trying to do, especially when those cards enter your hand at random. Dominion is fine because in any given game there are only 10 cards (except when that's not true but you know), but this is a problem I have with a lot of other deckbuilders.
3. The iconography on the cards is hard to understand. I won't say "the iconography is bad", because it's reasonably compact and I suspect that in theory, one could learn what all the different icons mean, and then know what every card does without ever reading their text; and that's what iconography should accomplish. But the cards do so many different things that there is no concise language of icons that can express all the different things clearly, so I always have to read the text on every card anyway, meaning the icons don't do all that much. (They do help a little, because after I've familiarized myself with a card the icons help remind me which card it is.)
In any game where I have to read text on cards, it adds a whole extra step to any process that involves my thinking about cards. In a game with very clear iconography the process goes something like "glance at every card to understand what it does", "think about every card and how they fit into my game", and then "make a decision about what card(s) I want"; whereas if I have to actually read the cards, I need to prepend a step "read every card and make sure I understand it". In a game (like Terraforming Mars) where you have to make decisions about cards all the time, adding that first (relatively long) step slows the game way down. I agree with Donald about the extra step of choosing which cards to keep and which to discard should not exist; the slowness of that is amplified by my having to read each card.
Again, Dominion is relatively fine here because you only have to read 10 cards in any given game.
4. There are a few cards that target a single other player, of your choice. Why would they do that? I think by definition Terraforming Mars cannot be considered a Euro specifically because of these cards.
5. I find the game impossible to internalize. This is a problem I have with a lot of heavier games, but Terraforming Mars is I think lighter than most games that I have trouble internalizing. I cannot listen to a full teach of the game and then have a sense of what I'm trying to do. And I suspect that this boils down to, you have no idea what you're doing until you've seen all the cards. This is largely because the function of the resources is determined mostly by cards. Also point salad automatically makes a game much harder to internalize (this is the big drawback of point salad to me), but there are plenty of other point salad games where I can listen to a full teach and then feel like I have at least some sense of what I'm doing. Someone can teach you the rules of Terraforming Mars and by the end of it it still feels completely abstract, and your moves in the early game are uninformed and probably unproductive because you can't predict how the cards you see later are going to affect things.
So I've come to the conclusion that the appeal of Terraforming Mars must be entirely thematic. In particular, it seems to me like the designer(s) didn't put much thought into making a mechanically interesting game. Instead, their goal in the design seems to have been to find mechanics that they could fit to the theme. This would explain the single-player targeting cards (you definitely wouldn't include those cards in an otherwise non-political game if you weren't thinking about theme). I get that there are people who care about theme more than I do, but for me a good theme doesn't make up for bad mechanics.