Played about 25 hours so far, and while I'm not near done yet, my general feelings amount to this:
1. Far too many fetch quests. While both M1 and 2 had their share of these, it feels like the ratio of fetch-to-main quests is much, much higher, and also harder to complete (or simply harder to find). I realize Bioware was going for a less direct approach to make the quests seem more like overheard needs (to what effect, I'm unsure), but having characters you could actually target to socialize with was far more useful.
2. The modelling, both for many main characters and for most supporting roles, sucks. ME1 and 2 made pretty much everyone look unique if they weren't NPCs. But with ME3 you have characters like Traynor who seems so... generic, and then far too many NPCs like Brynn who look really close to FemShep combinations. Especially bad have been characters returning like Kelly and Jacob, who looks absolutely awful next to their ME2 counterparts. Jacob actually looks old and feeble, while Kelly looks like she got punched in the face.
3. Technical problems galore. I've had Shep's arms glitch out while she's using her headset to communicate in some scenes, and Mordin's "tower scene" had his head glitch out. Really ruined the effect in both cases. During Jacob's sidequest, Tali's model didn't load, etc etc etc.
4. Related to point 1, the non-targeted interactions really ruin some moments. I feel like Bioware was being especially lazy here, because a lot of small quests in 1 and 2 offered some emotional drama even from really minor characters. Now? It's all just dialogue and flapping mouths. So when you relay news that so-and-so's partner died on some planet somewhere, sure you hear a choked up voice, but you don't *see* anything. It really kills the mood. By far the worst offender is finding Liara's father and getting them to talk.
5. It IS still a good game. But it's the weakest in the trilogy. I'm enjoying the main plot, but there aren't any storytelling improvements over ME2. It feels like they downgraded from ME2 in many ways. ME1 had better storytelling and characterization, while ME2 had better modelling and gameplay. While ME3 has some great moments (meeting Jack again, the Krogan genophage sidestory conclusion, Thane's conclusion), it's also lacking in so many other ways.