He has a MUCH shorter range of emotions, Tennant being able to pull off pretty much any emotion completely convincingly (when he was sad he looked like a kicked puppy, when he was angry I could see why the Daleks were scared), something Smith just doesn't have. Smith is happy and funny near constantly with some sad, but his sad doesn't have the same effect. Personally I'm not really fond of Smith's Doctor or his stories, they're too in your face look at the Doctor action action explosion climax disappointing ending well that was nice. The appeal of Smith is first time watching, and in that respect he can be better, but because of the format I just can't rewatch Smith like I can Tennant, and there's too much of a focus on being big and climactic for me to get into the episodes. Also, Smith eps have a tendency to be unreasonably copout-y and disappointing. Smith relies on the sonic screwdriver more than any other doctor, and most of his enemies are defeated using the power of love in some way whether it makes sense or not. Tennant wasn't my favourite, but he was amazing.
Well I liked David Tennant but I felt he took himself too seriously and spent too much time talking about how the human race are babies and naive and how they need intergalactic protection and shouldn't be attacked by aliens. I like the way Matt Smith is light hearted and amusing... I liked them both. Just for different reasons.