1. 1990 has emotional arc but can be over-dramatic/cheesy; 2012 is consistently flat.
2. Three-boobs shows up in both.
3. 1990 has mutants; 2012 has blue-collar workers
4. 1990, the issue is lack of air; 2012, it's lack of space.
5. 1990, the environment of Mars is a key plot point; 2012, the Lift (traversing the core) is a key plot point.
6. 1990, the doctor sweats and shows fear, thus suggesting he's lying, and Quaid shoots him; 2012, Malina sheds a tear, but... what does THAT mean? Even in Quaid's dream, it would have made sense for her to be scared, as he was supposed to kill her. A dream Malina would have cried as much as a real one. So I think 2012 is more murky, it's hard to know if it's real or fake.
7. 1990, Quatto tells Quaid that his identity is not in what he was, it's in what he does now; 2012, Quatto does the same -- Quaid is not defined by his past, but by his present.
8. 1990, there's Lori and there's Richter; 2012, there's just Lori playing both roles.
9. 1990, cops that look like mall cops with swat armor; 2012, robots that look like wanna stormtroopers.
10. 1990, Quaid pulls a bug out his nose; 2012, Quaid pulls the phonepad out of the innards of his hand.
11. 1990, Quaid is disguised as big colorful woman; 2012, there is a similar woman in the foreground, but Quaid is someone else; both use a device that disguises them but that malfunctions.
12. 1990, a LOT of music and kind of over-the-top, in an Airplane sense; 2012, the music blends together without being memorable at all.
Also, I have to say -- many critics bitched and said 2012 gave the mystery away too soon, but it did no worse than 1990, where the doctors in the lab made it clear that they never even injected Quaid with memories, so obviously all of it was already in him. Anyone bitching about this just needs to watch the first movie again -- both of them had about the same degree of transparency and/or uncertainty.