From memory:

• In ME1 Liara has different voice lines, depending on when you decide to recruit her.
• In ME2 if you wait too long before the final mission, (most of) your crew dies which will affect the story of the last game.
• In ME2 you have to talk to your crew regularly to get them to unlock special options or abilities.
• Talk to Conrad Verner all the time in 1 and 2 and you’ll get a reward in 3.