Reckon the DnD class system is fantastic for a certain type of game: If you want a 'dungeon crawl', it's easy to do - way more so than any other system I can think of. Clearly defined needs for a party and lots of straightforward archetypes to fill all the niches. I'd go as far as saying it's a great introduction to RPGs. Certainly the sort of thing that got me playing.
Before anyone pounces, I'm NOT saying you can't do an awful lot more with the system - particularly with some splat books, house rules and/or imagination. But it does make that sort of game much easier to get right into?
I like flexible point buy systems a lot - although my early experiences of them were much like Rystil Arden above - a group of horrendously overpowered PCs. As times gone by, we figured on making them with the GM, just to make sure everyones ideas of "balanced for play" are the same. Playing an excellent DC heroes campaign at the moment - we gave superhero concepts to the GM and he went off and twisted the mechanics round - a great way to make sure everyone fits together well.
Think adding a point buy class system would be fine for DnD - but I'd rather they did it in a separate splat book. Avoid 'muddying the waters' for some newer gamers?
As an example, point buy classes would likely ruin the kind of game my 12 y/o cousin and his mates have been playing. Unless someone came up with a totally balanced point buy, which I can't see happening.