Dr. Mordin Solus is my favorite addition in ME2. Like others have said, the depth of character is amazing. Mordin has an incredibly unique personality, and I liked how his loyalty mission showed some hidden depths. It also took the chance to tackle some really 'heavy' issues of morality. Finding a video game that is willing to discuss very complicated themes such as biological experimentation is hard, but that's why I play mass effect.
I think of Mordin as being something of a foil to Liara (Liara and Wrex were my favorite ME1 characters--like Mordin, I thought they were both unique, plus the hidden depths revealed was a big bonus--I actually wanted to talk with Wrex, Liara, and Mordin after missions). Liara is quieter, reserved, very shy talking with Shepard. However, Mordin is much more confident. Of course, they both kicked ass in combat, I took Liara and Mordin to the final boss (Liara's biotics were very useful, and I wanted to be 100% certain Mordin would survive the suicide mission, so I brought him with me for the entire thing so I could protect him.). I certainly hope that they both come back for ME3
Also, Mordin's 'romance path' was hilarious.