13:8 Here Paul uses the word fulfilled (plero-oo) and the law (nomos)...Paul also uses the words together in 8:4 in conjunction with the Spirit's work in us. In fact, Paul uses these words together in Galatians 5:14; the translations there say the law is "summed up" in this one command. One more note -- the word fulfilled here is in the perfect tense, which means that there is nothing more that has to be done, that is completely finished and remains finished.
13:9 The big word here is "anakephalaio-oo"; literally again-headed or recapitulate. It means bring or sum together. It is the word that Paul uses in Ephesians to talk about how things were summed up and brought together in Christ. Here is Romans Paul does not say all things are summed together in Christ. Instead he uses indicates that all the commandments are summed up in this Word (logos; it is not commandment). (Technical grammatical note; look at the gender of possible antecedents to see why it is Word and not commandment). It is curious why Paul switches back to logos language here, perhaps again connecting the word, the law and the work of Christ. Also interesting is that Paul in Galatians 5:14 uses this same language of the fulfillment coming in this one Word (logos), instead of saying the law is fulfilled in one commandment. This is too easy of a Lutheran out, but I find it fascinating the neither the law nor the commandments are fulfilled or summed up in a law, but rather the word.
13:12 Paul encourages people to put on the "armor" of light. The word here for armor is "hoplov," which is what Paul discusses in chapter 6 of Romans. Hoplites, in fact, are the name of the common soliders in ancient Greece (http://en.wikipedia.org/wiki/Hoplite)
13:13 This verse is the verse that sent St. Augustine to God! Interestingly, the word here for "live honorably," is euschemonoos. The word in here -- schema -- is the word we heard in Romans 12:2, not to be conformed to the scheme of this world.
13:14 Now we find out who the armor of light is -- Jesus Christ. It is the same verb that is used in 13:12 for put on the armor of the light.