The defense is going to be an issue this season but it's not the fact that we're running a 3-4. It's a number of factors that include the following:
Seth Payne, coming back and getting back to form (DeLoach may be filling in admirably for Seth but he's not Seth. He's never going to be Seth)
Gary Walker getting back into the groove. A much smaller step up for Gary than Seth but he still missed significant time last season.
Robaire Smith getting fully integrated into what we're doing and in sync with the rest of the defensive line.
Starting a new rookie linebacker (Babbin) who's learning a new position
Starting a new rookie cornerback (Robinson) who's not learning a new position but who can single handedly give up a score when he makes a mistake.
Starting Marcus Coleman at Safety. Not a rookie but a new position.
All of these things are going to take a few games (at least) to get worked out. Remember that whole "gel" thing? They need to and until they do they aren't going to be playing well. At times they'll play great and at other times they'll blow it. Come week 8 or 9 when the defense is really on the same page without thinking about what they're doing many who are worried about whether or not the 3-4 is viable will be talking about how they're finally playing it like it's meant to be played. As if this is a result of the way the Texans are trying to play the 3-4 and not a result of the changes they've made to the personnel.
Fitz makes bad decisions. That's not going to change.