I liked Leto's performance for the most part. There were a couple spots that I thought were over the top or not very good, in terms of his actual delivery/performance the 3 that stood at as bad to me were...
- His facial expressions during the Batmobile chase were pretty awful
- His over the top delivery of the last sentence in the "Infamous Harley Quinn" line, the first sentences were good, then he flares up and uses silly hand gestures and changes his voice a bit, it was too much
- The "Kiss the Ring" moment, the scene itself is good, but that particular part, is so cliche and un-original, just stupid. Though that's not really a performance thing, he sold it fine, it's just a stupid played out cliched gesture
The writing wasn't particularly great.. the "Hunka Hunka" line is pretty bad writing but I think his delivery of that line was good enough that it didn't sound as horrible as it could have been.
The characterization is a bit off, I didn't like the heavy Mob Boss tone or the romantic theme of his relationship with Harley, but that's not Leto's fault, that's the storyline/director/writing. Obviously we know now they removed alot of the abusive relationship stuff, that was a mistake. There has always been a bit of a Mob boss theme within the Joker, but they went a step further than ever before and it takes away from the uniqueness of the character.
Overall I thought his voice, mannerisms and laugh were very good. I had no problem at all with growling/purring, though I don't want him doing it every other scene, but it didn't bother me in general, actually I thought it was good in the Ike scene. The "Hurt you really, really bad", "Chemical Bath" and Arkham/Harleen scenes I thought he was great in.
As for his look, just dump the damaged tat and ALL the jewelry, give him the purple suit... Do that, and his look is very good. I like the lack of eyebrows and I've even learned to like the grill (though they should make it a bit more form fitting). The rest of the tats, whatever, just don't constantly have him shirtless/open shirted and I don't mind them. The purple suit is a must. But all the flashy cars/clothes and jewelry is just awful, really really awful.
I thought he was good overall and has a enormous amount of potential if they make some tweaks here and there. The worst part of it was the decision making for the characterization for me. Turning him into a generic millennial gangsta/mob boss was just stupid. Leto's performance itself was good overall IMO.