What he has achieved (successful everywhere he has worked), how he has gone about achieving it (the odds always being stacked against him, yet when he left everyone was singing his praises), what his ethos is, how hungry he is, what style of football does he play, and how will he approach the job at Spurs. And I would also think what intangibles he brings to the table, where I think he is head and shoulders above anyone else.
It's not just whether he has managed in a top league, if that's what you're thinking about. There are so many other things that make a manager a good one.