First, we usually don't open gifts at the party. When we have. the kids got distracted by the "one perfect gift" and then every other gift just seemed rushed so they could get back to the thing they loved the best, thus seeming rude, when really they were just being normal kids. We open at home, then my kids write thank you cards as we put each one away. I also use that time to "hide" a few of them, so that I can bring one out every few weeks or so, so they have something new to play with.
Secondly, more in response to the other responders than to you, it is very common here for the kids to invite their entire class. We have done the same for both of our kids. Their schools have rules that they can only help facilitate invites if they are for everyone, so we invite all 26 kids, knowing that 10 - 13 or so will come. Add to that their siblings (we always include "Siblings Welcome" on invites), family, neighborhood friends, and so on and 30 gifts is not out of the ordinary for them. I don't see it as being too much or just wanting gifts, etc. When looking at inviting the whole class, if kids are left out, they feel bad when others are talking about the party, plus those 25 kids are the ones who my children socialize with more than anyone else during the week. They are there 8 hours a day, 5 days a week, and we are still at the point where they are all "friends". I'm sure as they get older, we will just invite smaller groups to the parties, but right now when they are all friends, why not?? Just my 2 cents. :)