I haven't read it in superfine detail, so here's my first impressions. If I got anything wrong, please let me know--I'd rather be wrong about some of the changes I don't like!
1. Grenadiers - you can still get them (as an upgrade to veteran squads), but they are armed with lasguns, not hellguns. This in itself doesn't matter so much. But what I would have liked to see is the ability to get carapace armor for my infantry and keep them within the normal organization (buy them in platoons with command squads, etc). As it is, you can only get them in single squads.
2. Orders and Vox-casters - The concept of orders seems pretty cool, and I look forward to trying them out. The ranges seem a little short to me, but they make a certain amount of sense--if you have to shout orders across a noisy battlefield, your range will be limited. Which leads us to radios...I mean vox-casters. The entire purpose of these devices is to allow communication over large distances, so they should increase the command radius of an officer, provided that both the command squad and the squad being commanded each has a vox-caster. You would think so, but no. They allow a re-roll if the leadership test fails. This leads to situations like this:
Forgive me, but that's just stupid. If I was writing the rule, I wouldn't allow a re-roll, but make the orders radius unlimited (or at least extended). If this is unbalancing, then raise the price of the vox-caster. To 100 points, for all I care...just make the rule make some sense.
I suppose it could be argued that vox-caster ranges would be limited on the battlefield because of enemy jamming etc. Maybe. But if so, jamming also makes radio communications more difficult to understand--so the rule on re-rolling a failed leadership test doesn't make sense either.
Anyway, I hate this rule and I find it very frustrating. I couldn't care less about it "hurting" the effectiveness of the army--it's the fact that it doesn't make sense to me that I find it annoying.
3. Sergeants - Apparently these days Sergeants are stuck with pistols. The only exceptions are Stormtrooper Sergeants, who can get hellguns or bolters, and Veteran Sergeants, who can get shotguns. Shotguns but not lasguns? I could live with sergeants not getting exotic things like stormbolters, etc., despite many of my models being converted to have them. What I find baffling is that they can't get lasguns--the "universal" weapon of the Imperial Guard. That just makes no sense to me at all.
All in all, it's nice to see an updated codex, and I'll enjoy trying out some of the new things in it. But some of the changes irk me. Since I only play friendly games anyway, I'll do what I usually do--ignore them. With my opponent's consent, of course. In practice, I'll probably allow sergeants to carry lasguns if not bolters/stormbolters, and I'll probably follow the vox-caster rule as written, but grumble about it a lot.