The +1 is certainly not spectacular, but those +1/+1 counters do add up, and he starts with 4 loyalty right off anyhow. The -3 is probably the best part here, as you can use it immediately without losing him and give something flying and double strike. Getting in for a quick few damage is always a good thing, and since he's only 3 mana to cast, you could easily play 3 or 4 of him in a deck easily. The ultimate ability is a -8, which means you'll likely rarely use it, but if you do get it off, the 2/2 Cat tokens could become quite relevant. It's all about keeping your life total high enough to make it relevant.
The Commander applications are quite obvious. The Standard applications are not quite as obvious, but there's little doubt that he's going to be very playable in Limited and will definitely be played in aggressive mono-white decks (of which there are likely to be many after this Core Set is released).
At the very least, this Ajani going to be a target for removal. Your opponent may likely have to deal with him instead of one of your bigger threats later. He's too big of a threat to not remove as soon as possible, as he's going to pump one of your guys every turn he's out there. I think he'll always be worth the 3 mana, even if he's removed right away, because that's one less removal card you have to worry about for an easily replaceable card. Not to mention, as long as Sun Titan stays in Standard, Caller of the Pride is going to come back and say hello at least once in a game.
Is he incredibly game-breaking? Not really. But he can certainly help give your army an extra push, and sometimes, that's all you need.