The basic CR system tries to assume you've got some good magic items for your level and such. Which means that in a low wealth game, the DM has to compensate by lowering the CR on monsters a bit so you can actually do something decent. I have a feeling your current DM likely hasn't adjusted things properly for that. For AC to actually be a decent defense it typically needs to be at least 20 + Character Level, which it doesn't seem like you'll be getting to any time soon.
What books/other sources are available to you?
The feat N'raac mentioned is
Improved Combat Expertise. It might help you out on the AC front.
Don't try to rely too much on crits unless you know the kinds of monsters you'll go against aren't immune to them. As you level up, you'll tend to come across more monsters that are immune to crits and you likely won't have access to the items needed to let you crit against them (there are weapon augment crystals in the Magic Item Compendium that'll let you crit against undead and constructs, but they cost 10,000g and 6,000, respectively.)
I expect you've got an inkling to go for more damage and accuracy on your scythe, so make sure you take the
Melee Weapon Mastery feat before you even think about Greater Weapon Focus or Greater Weapon Specialization.
Also note the
Bounding Assault feat.
I have a very strong feeling your DM won't allow it, but if your DM miraculously does allow Tome of Battle, a level of Crusader will get you the
Thicket of Blades stance, and you'll probably want Combat Reflexes with it to get more AoOs. Yes, it's a 3rd level stance, but you qualify for it due to already having several levels.
If you can retrain feats via the PHB2 rules (page 191), I'd probably recommend switching Improved Critical for Combat Reflexes. You have at most a 10% chance to crit with Improved Crit, and since you're almost certainly not the primary damage dealer trying to go for crits won't really work for you. Though it sadly doesn't work too well for many in the first place.