I was actually really excited to see Slayer for the first time 'cos I'd seen Megadeth quite a few times in the past (with an older lineup, though) and it was just.... well disappointing. I think the mix really highlighted how average they all are (including Lombardo) as musicians. Now that's normally not an issue and I'm not one of those prog snobs who hate bands who aren't incredible musicians, far from it - I'd rather a band play sloppy and put on a top show, or be raw and have a great energy happening... but when a band just stands there looking bored and still sounds sloppy... it's just not good.
What everyone said was right - it was sloppy WAYYYY loud kick drums, no snare for most of the show, quiet cymbals and a couple of the middle toms were gone in the mix, no bass guitar, Kerry up WAYYY loud and no Jeff at all and a real half-assed vocal performance from Tom (although in his defence, if he was starting to get sick, that's very forgivable - I've been there and it's a sucky place to be as a vocalist. Big kudos in that case for doing as well as he did). They all really just didn't look like they wanted to be there at all, which made it all seem even worse than just the bad mix.
By sheer contrast, Megadeth was just non-stop BANG BANG BANG BANG one song after another, pausing to say thanks, and then smashing through some more. Top musicianship from everyone (Dave's voice was a bit shaky on some of the older higher vocal parts though), and the mix was good, if a bit quiet.
I can't see how anyone could see those shows differently to that... this isn't just a Megadeth fan talking, I REALLY REALLY wanted to get blown away by Slayer because I've been a fan since 1986, but it was just poor even by a second tier Aussie local band standard, and I've seen a LOT of bands over the last 20 years...!