While looking over TV Tropes amazing page on Mass Effect 2 characters, I noticed a bit of a pattern for a lot of the characters. There are some pretty convenient back doors to explain why several won't be playable characters in Mass Effect 3. Here's just a short list of some of the people you might be missing whenever Shepard finishes his fight with the Reapers. 
 
Mordin Solus: Perhaps the most heartbreaking of the bunch, but he's also implied to be 38, which is really old for Salarians. Average life span is 40 years old, and those final years are very difficult ones. If any amount of time were to pass between ME2 and ME3, he would likely not be in fighting shape. Furthermore, he is by far the hardest character to get through the Suicide Mission portion alive, possibly one way that BioWare tried to soften the blow of him not showing up again later. Unfortunately, they clearly fucked up by making the fragile character possibly the best character in the entire game. 
 
Samara: Also very old, even by Asari standards. She also has dedicated herself to the cause of the Justicars, and would likely be hard pressed to leave that life behind. She does tell a Paragon Shepard that she will always be a friend and will support him/her when possible, but that doesn't sound like a request to remain a Normandy member. Conversely, Samara will tell a Renegade Shepard that when this mission is over, if they ever cross again she won't hestitate to kill her/him. Add the fact that she can be betrayed for her psychotic sex-vampire daughter, and you have on possibility out of three that makes her a viable return character. This makes it fairly likely she'll appear as a cameo, if at all, in the next game. 
 
Morinth: As a fairly deeply hidden character that requires some focused Renegade play, doubt that the game scientists at BioWare would find it worthwhile to include substantial material for her in the game. Besides, even by the Normandy's rather...colorful crew, Morinth is a serial killing sociopath. I doubt she really even has much interest in saving the world. Once it became clear she ain't getting no Shep-love, I doubt it would be an issue. Almost definitely not coming back. 
 
Thane: Of those likely not to return, Thane is the most likely to return simply for being a love interest, and BioWare seeming very keen on that being very Important for the overall narrative thrust of the story. Still, he is dying of a disease of which there is no known cure, and has personally come to terms with his death, especially after reconciling with his son. The Lair of the Shadow Broker information suggests that some form of treatment is nearing usability though, and I know that BioWare at least is very fond of the character. So...really, probably doesn't belong on this list, but I'd be unsurprised to see it go either way. 
 
Zaeed Messani/Kasumi Goto: As DLC characters, it is unlikely that they would be graduated up to being main game content. The PS3 re-release with the content baked in might make this more likely, but I'd not place any money on them returning. Which is a shame, at lease for Kasumi. I like her quite a bit, mostly because she's not as opressively grim as most of the members of the Normandy tend to be at times. 

Zaeed's a bad ass, I will miss him dearly if he doesn't show up.
Samara, Morinth and Thane never interested me much, but by logical standards we won't see them back.
 
I hope Mordin at least accompanies us in the first bit of the game, he's the best character to combine with the genophage cure story and it would be a great moment to hold a funeral or something like it for him.
I just hope we get to keep Grunt and Jack, altho Jack would be a long stretch but who knows, maybe she found some good in her torn heart and decides to help Shepard.

If they are going to limit the cast of your squad then because this is the final game they need to be involved in the story somehow, they really pissed off those who romanced with ashley and kaiden and with the possibilities of who lives and dies, they can't half ass it like in 2. and have a 3 minute conversation and have it as that

Out of those Mordin and Thane better reappear.

I'll also miss Zaeed and Mordin. And though I liked Thane, his story necessitates his death.

I would bet my life on Thane reappearing in Mass Effect 3. He was a huge figurehead for the sequel's marketing thrust, and became a sort of "Garrus-ian" badass without, you know, being Garrus. 

I could totally see a Thane suicide scene in which he gives the last of his life to save Shepard and the crew. He'll most definitely reappear, although I doubt he'll be a party member. 
Mordin will probably also have a cameo, but he probably won't play a major role, unless to invent some sort of defense against the reapers. 
As for the rest, yeah, they probably won't appear, which I would be totally fine with. As long as I can role with Garrus, I'm good. 

" @Dany Actually, there was a very good reason every character who was a playable character in 1 isn't one in 2. Any character that was a possible love interest in 1, needs to be alive for the third game. Because of the structure of the Suicide Mission, BioWare didn't want to risk the potential issue of having them die, so Liara, Kaiden and Ashley couldn't be playable. Wrex, meanwhile, could possibly be dusted in Mass Effect 1, so him being a playable character in 2 would have created a full recruitment and loyalty mission that an unknown percentage of players would have never seen. That said, that makes his appearance in ME3 rather dubious as well.  It should also be noted that in love interests from ME2 no such safety net, seeing how any of them can possibly not make it out alive of the suicide mission. "

Man I want Thane to come back so bad.

" @HubrisRanger said:

" @Dany Actually, there was a very good reason every character who was a playable character in 1 isn't one in 2. Any character that was a possible love interest in 1, needs to be alive for the third game. Because of the structure of the Suicide Mission, BioWare didn't want to risk the potential issue of having them die, so Liara, Kaiden and Ashley couldn't be playable. Wrex, meanwhile, could possibly be dusted in Mass Effect 1, so him being a playable character in 2 would have created a full recruitment and loyalty mission that an unknown percentage of players would have never seen. That said, that makes his appearance in ME3 rather dubious as well.  It should also be noted that in love interests from ME2 no such safety net, seeing how any of them can possibly not make it out alive of the suicide mission. "

I understand everything behind what they did but I felt it was poor way to handle it. "

Zaeed and Kasumi are under Cerberus Contracts - and I think its made pretty clear that they were hired just for the collector problem - but still, I have a hard time believing in any good reason for the characters to stop fighting for the cause.  
 
Samara seems to lack a good motive as well, she did swear an oath to Shepard - although I don't think it was clear when Shepard's "mission" would be complete, still, I wouldn't be too upset about this loss.  
 
Honestly though, I'm more beat up about losing ME2 characters than Kaidan or Ashley, that might be unfair to them - but the way they were written in ME2 just makes me so god damn frustrated - and each seem kinda limited in actual usefulness - Kaidan is almost a clone of Miranda/Jacob - Ashley is pure combat - which Zaeed and Grunt have covered.  d