He unsuccessfully challenged Gordon Brown a couple of times, which was very brave of him. He's way more intelligent than Corbyn (including emotional intelligence) and understands economics and balance sheets, unlike almost everyone else on the left. He's a Trotskyist, though, whereas the string-pullers working Corbyn are, basically, Stalinists and the two lots loathe each other. He's also more pragmatic than many on the Far Left without totally compromising his principles, and didn't speak to Livingstone for ten years, which earns him great respect in my eyes. He'd have shut down both the anti-semitic and pro-Hamas/Hezbollah charges quite quickly if he'd been in charge, even the pro-IRA stuff, as he's already both apologised and explained the reasons behind the necessity of talks with them. Being someone just a couple of years older than Corbyn/McDonnell, though, I'd say it's a huge mistake leading a party that late in life.
P.S. Late news - McDonnell has just announced he's standing down from the shadow cabinet - now Corbyn needs to announce something similar, with immediate effect.