It is a pita figuring out the ins-and-outs of a new system. Part of the problem with Canon is they have so many lenses it can be tough to figure out a "coherent" system. I've done a lot of reading and there are some lenses that seem to be consensus "must haves."
Right now I have the 24-105/4 IS, 70-200/4 IS (awesome lens), and 50/1.4. That gives me pretty good coverage, but I want some faster glass. The other problem with Canon (and Nikon...and any FF lens) is that fast glass gets big and heavy. Pentax spoils you with the FA ltd primes. But just like the 50-135* was "prime good", the 70-200/4 IS is the same. I would love to get a full complement of L primes, but some of them are just too damn expensive, and the cheaper/slower variants are pretty close (eg 50/1.2 vs 50/1.4). I probably will add the 135/2 L as I'd like something long/fast, and maybe the 24/1.4 or 35/1.4.
Losing IS was a concern, and one reason I went with two zooms with lens IS. So far the 50/1.4 has been fine though, and I've been shooting the same bad light and slow shutter speeds I did with the K20d. Maybe keeping the number of cups of coffee per day down is working