One of the biggest reasons for my initially buying this particular Zeppelin album owed much to curiosity; as in curiosity as to why so many Zep fans--nevermind the critics--seemed to trash it so much. Having been previously inundated with Zeppelin I, II and the almighty fourth album, this was indeed "different". But the thing I came to appreciate about Zeppelin over time was how 'different' a lot of their latter days output was and how their musical style progressed over the course of eight studio albums.
"The Brown Bomber" (Zeppelin II) and "Zoso" are great records, but I'm pretty sure I wouldn't be as big a LZ fan as I am now if every single album sounded like them, or if Plant & company felt they needed to endlessly recycle the riff from "Whole Lotta Love" to keep their "true fans" happy and never try to expand beyond nicking old blues numbers.
For one thing, the much carped about use of synthesizers featured on "In Through the Out Door" never once bothered me; it wasn't like Zeppelin never used them on a song before ("No Quarter" anyone?); having Jones back on the keyboards/piano for this one makes for a refreshing variety amongst all the tracks, an ingredient that was sorely lacking from "Presence".
I'll just finish this by simply stating that ITTOD is by no means an album to be ashamed of. For me personally, it's at the very top of the list along with "Houses of the Holy" and "Zoso". Times change and so do many truly great musicians over the course of their careers; Led Zeppelin was no different.