Myself, I try to enjoy something for what IT is, rather than trying to compare it with the series it is part of.
If it's part of a series, it is inevitable that it will draw comparisons. And one bad movie in a series can leave a bad taste in people's mouths, as we've seen with many after TPM.
The thing is.....everyone wants to see another great Indy flick, but they don't want it to suck. After observing the recent work by Ford, Lucas and Spielberg, people don't think it will be great. There are other reasons too, too much time has past, Ford is older and may not be up to the task, the fact that's been in development hell for years, Ford's motivation for wanting to do one, etc.
There wasn't any of these factors back when the original three were made, so naturally many of us are skeptical. It's not that we don't want another one, Adam....we just want a great one and if we can't get a great one, we'd rather not have one.
Also, to many of us, Indiana Jones was as big of a deal as Star Wars back in the day. I saw all three in the theaters and they rocked it hard, man!!!