I've been thinking a lot lately about web pages that do too much in a single page or what I call the "everything page" and what it really costs to develop and maintain one. It's a topic that Trent and I have also been talking about in terms of web development being developing sites as a collection of pages (e.g. your typical shop cart checkout process) versus web applications that function more like a traditional application where a main window is represented by a single page (e.g. the Gmail interface).
It seems that in the majority of web applications I have worked on the paradigm is usually several pages that are smaller in complexity and then one or two pages that are real doozies. Often times the complex pages don't start out that way but as features get added, things have a tendency to compound. Whether they start out that way or end up that way, I'd argue that getting a handle on that complexity and finding a way to reduce or manage it is critical to keeping down the cost of development and future support.
So how do we deal with these monster everything pages to make sure they don't cost us an arm and a leg?
The other approach I would mention is related but one that is often a harder sell. You should consider how you can break that page apart into multiple pages. A product owner or business stakeholder might make an assumption that having everything happen on a single page is just as easy as having a multiple step process (with all that "annoying clicking" to navigate). What is the real cost of cramming a ton of functionality into an everything page versus spreading it out into smaller pieces and do the stakeholders know? For example, if you have 4 pages that each take 5 hours done separately, would you consider putting them into a single page if it instead took 40 hours and all future maintenance took twice as long? Depending on the UX demand, the answer might be yes, but I don't think this cost gets factored into the decision often enough.
The most important thing you can do though is to recognize right away when you have an everything page in the works so that you can start to think about ways to manage the complexity and keep the true cost of the page under control.
Q: What did the Zen Buddhist say to the hot dog vendor?
A: Make me one with everything.