Hard to re-watch, hmm... I don't think my opinion of any of the DC shows has dramatically changed for the worse when I've left them for a while and then come back to them. I suppose JL season 1 seemed more mediocre when I was watching it on DVD than it did when it first aired in the UK... Also, I have to admit that a couple of early B:TAS episodes ("Christmas With the Joker" being one of them) were a bit tough to watch when I got the DVDs and made me wonder if I'd made a serious tactical error in buying the three DVD volumes. But then of course I checked out more and more episodes and was soon very happy with the purchases.
I have to admit that I don't really the feel the urge to re-watch YJ. I know I will sometime, having got the DVDs and watched them all last year, but shows that are highly serialised tend to lack the 'safe' structure of a clear beginning, middle and end (a structure which standalone episodes and multi-parters have). It makes the prospect of going back, post-series, to say the middle of the first season or a few episodes into the second season a bit uninteresting because when you watch those episodes you know you're not getting a full story - just a part of bigger tapestry which began before that episode and finished (or was meant to finish) much later on. Whereas you can jump on board B:TAS or Batman Beyond at pretty much any time and the aforementioned problem isn't in evidence.
I also found many episodes of Batman: The Brave and the Bold a bit tough to re-watch (never mind the ones that just bored me on the first viewing). The charm the episodes had when they debuted was the cast of characters and/or setting unique to that episode. Take away the 'surprise factor' of what's coming next, where you've now seen all the episodes and you know who all the guest stars were, and the fun has gone out of it. While there were a handful of excellent episodes, the show was really just style over substance and the possibilities for future team-ups were what mainly fuelled my interest in the show.
The Batman also falls into the 'no plans to re-watch' category.
Teen Titans season 1 was and no doubt still is tough to watch. It was easily the worst season of the show IMO, with only "Nevermore" and "Apprentice part 2" standing out. The other episodes were either just plain bad or cliched, and cliched plots are only redeemed when you have a lot of fun with them - as "Car Trouble" did, to an extent, admittedly. Seasons 2-5 were much better and far less boring.
Some shows get more tolerable with time though. When S:TAS aired in the UK, they only showed the first season and I was bored by nearly every episode (except "Stolen Memories", which I only seemed to catch glimpses of whenever it was on). I took a gamble with the DVDs, but it was a gamble that payed off and I enjoyed many of the episodes, including first season episodes like "Fun and Games", "Feeding Time" and "My Girl".