As for tires, it really comes down to what sort of roads you're going to be commuting on. I ride about 20 miles each way, spring through fall (yes, I wuss out during the winter months) three or four times a week. My commute is on good roads through suburbia, so I use 700x23 tires on my regular road bike. Once the light starts to fade, I put lights on the bike. If you're riding on crappy potholed roads with various debris (broken glass, gravel, etc) with regularity -- wider tires are nice.

As for components... As the componentry moves "up" in Shimano's line (i.e, 105 > Tiagra > Sora), it gets lighter and, to some degree, prettier. The bearings and springs and other internal "guts" of things might be made of higher quality / more durable materials as well... The brake pads themselves are a larger determiner of stopping distance than the brakes (i.e., the metal part) is. Likewise, how clean you keep your chain is a better determiner of how well things will shift than the name stamped on the derailleur and shifter.