Morgon - I second Serren's recommendation for Paradigm speakers. I have 2 sets of them and love their sound. But like I said, sound is a personal thing. Speakers that sound awesome to some, may not sound awesome to everyone. You absolutely should listen to them before you buy them! I have a paradigm sub as well (servo 15.. oh yeah and a PDR 10), but I have heard both the HSU and SVS subs and they kick arse for much less.
As far as the placement of your couch, don't worry too much about it. My couch is closer to my left rear, then my right rear. I compensated when I calibrated my system by picking my sweat spot seat and then setting the right a bit hotter than the left. That way the sound level is the same for both speakers when you are sitting in the sweet spot for your surround setup.
Tower speakers are great if you are going to be doing alot of 2 channel listening (music usually). That way you get the additional boost of the mid subs in the towers themselves (unless you go def-tech with subs in each). If you are going to be mainly using them for surround setups, you can really get away with a nice pair of smaller bookshelf type speakers. This is especially true if you pick up a nice sub. This way you have the extra room and your sub does all the heavy work for low end stuff.
As far as optical/hdmi, they are both digital connections so shouldn't differ. Well technically I suppose they are different, but in terms of quality, shouldn't be an issue. I think that HDMI may support more channels of audio or more bandwidth, but I'm not sure.
As far as pitfalls, if you do your homework, ask loads of questions here and elsewhere and audition your speakers, there shouldn't be any!
