I have a feeling the Rachni won't be useful until Mass 3. Letting wrex survive gets him to unite the Krogan towards curing their genetic blue balls, so the Krogan will probably at a later date once again be a galactic threat, and my guess is that the Rachni will be there to help you fight them off. It would be the exact inverse of the original situation, where the Krogan fought off the Rachni.

I'm hoping to be able to visit Zhu's Hope colony again, and see

I hope the Rachni play into Mass Effect 2. Like they help you in the end or something. 
 
Personally I hope, love choices play more of a role. Especially the choice between a human or Liara. Liara mentions that Asari get bad rep for 'doing it' with other species. Show me.

Oops, I killed the Rachni Queen.  Oh well.

I let the rachni queen live and now afraid that they will be the villains of ME:3.

" There's actually a full list of the choices that carry over.  I won't post them all here, but to mention a few..
 

SPOILER WARNING: Click here to reveal hidden content.

The Rachni Queen obviously carries over.  The whole scene with it in the first game was out of place, it didn't do anything and seemed to exist solely to build to something in the future.

And I'm pretty sure the more character based choices are going to be skipped over, which is most likely who Ashley, Kaiden, Wrex and Liara aren't on the party anymore.  The ones who died would have too much of an impact on the story, so instead if they lived, you're going to maybe get a few brief cut-scenes with them.   Ditto for the love stories, I'm sure they'll be acknowledged, but probably pretty briefly.  For me, my biggest choice was to let the council die, despite playing as a Paragon.  I thought taking the oppertunity to attack, rather than going in on a rescue mission and risking everything was the better choice.  So it'll be interesting to see how everyone reacts to having a completely human council. "

" @Zereta: I thought Asari get bad rep for doing it amongst their own species. It's considered good by them if you bang someone of another race. Hense why Liara wasn't like that much. She was a 'pure Asari' "

I doubt the Rachni choice will return in ME2. It probably won't have any effects until ME3.

"For me, my biggest choice was to let the council die, despite playing as a Paragon.  I thought taking the oppertunity to attack, rather than going in on a rescue mission and risking everything was the better choice.  So it'll be interesting to see how everyone reacts to having a completely human council. "

I let kaidan die, nothing important is going to happen. Rex might become a leader or ruler for his home planet and the coucil, who cares about them, it's going to be a military governement imo with the humans.

I let Ashley die because she was annoying and biggoted. 

There's no way letting the Rachni Queen live will turn out to be a good thing.  It's way more interesting to have your good deed come back and bite you.  That was definitely the vibe I got when I had to make that decision anyway.  They'll show up, you'll kill them all, somebody will pat you on the back and tell you that you did the right thing anyway.
 
I can see people being disappointed about how low-impact these 'choices' actually turn out to be.  I mean, just changing the Council would change absolutely everything that comes after, right?  So the solution is -as far as I can tell- to not have you working for the Council anymore.  If so, that doesn't really bode well for the less important choices - I'm expecting maybe a handful of scenes to play out slightly differently based on your choices.  
 
I can't really blame them if that is how it plays out - it's not like they could make 9 totally different versions of Mass Effect 2 - but they are hyping the shit out of the whole important choices / continuous trilogy thing.

The first rule of multi-path gaming is that the more things change, the more they stay the same.