I think it would be a good idea to talk about one of the most rule heavy part of most table top RPGs including L5R: Combat. The following is basically my general perspective on things and intended as a jumping off point for discussion. I'd like to focus on generalities of things rather than the mechanical specifics as we discuss this important part of the game.
Stances :
I like the concept of the 5 stances (Full Attack, Attack, Center, Defense and Full Defense) in l5r 4e. What I don't like is that using the Defense and Full Defense stances in may ways becomes "skip my turn" instead of an actual tactical choice that advances the combat. I'd like to see ways (such as rules for making counterattacks and defending to tire out opponents) that going into the Defense and Full Defense stances become meaningful tactical choices.
Kata
Kata in 4e drive me nuts. The name implies that they are a series of practiced forms used to build muscle memory and allow reflex like use of various movements (i.e. the one-two jab/cross combo in boxing). The description and individual names of the Kata treat them more like sub-styles of various schools. The actual rules have them working as mini-stances. I think this identity crisis along with the fact that learning Kata does not grant Insight is something that needs to be examined.
Maneuvers
I love the concept of Maneuvers. Being able to call raises in order to have more tactical options in combat is a great thing, but the requirement to call raises before the attack roll is made tends to leave them underutilized in a lot of cases. The variety of Maneuvers and their effects do not help either. Feinting for some reason increases damage and is potentially more effective than calling 3 Increased Damage raises. I also think that the some school techniques (for example the Daigotsu Bushi's IR5 Inhuman Assault technique) might be better if they were complex maneuvers that could be purchased for use.
Weapons
I kinda understand why 4e made the Katana the "best" weapon, but I don't really like that they did so. It sometimes feels like the choice is mostly "Does your school get greater benefits from not using a sword? No? Use a sword." I'd rather have the weapons somewhat comparable to each other with inherent advantages/disadvantages with the superiority of the Katana being something more cultural than mechanical.
Those are my thoughts for the start.