TL;DR: Mammoth software with tons of features are not uncommon nowadays. However, the interfaces for these software have not matured. They are not scalable enough to support efficient usage of the program's features. The traditional menu UI metaphor becomes very cluttered in this case. The solution: searchable user interfaces.
Good luck finding what you need in the menu-spaghetti.
The above picture should make amply clear the scalability problem with menu-driven user interfaces. It's a waste of the user's time if you make them manually look through the menus (or make them remember arcane keyboard shortcuts) for actions they want to perform, especially if your application can do a lot of things. The purpose of the interface is to make life easy for the user, and the GIMP's UI doesn't really make much of an effort in that direction. Sure, it organizes the gazillion "Filters" into neat categories, but the categorization could be subjective, or an action could belong under more than one category. We need a scalable interface. Fortunately, we do have a practical solution to menu-spaghetti.
Wouldn't it be amazing if, alongwith menus, your application gave you functionality to search through them, all the way down to individual items buried deep? That way you wouldn't have to go feature-hunting through the menus (or tax your brain with a key combination); you could merely search for what you wanted. And the icing on the cake would be if the search did fuzzy matching and took into account synonyms, various word forms, etc.
There are actually some well-known existing software which implement these "searchable user interfaces", or SUIs, as I like to call them.
First up, we have Google's zippy web browser, Google Chrome / Chromium.
Notice how Chrome searches even inside other UI elements.
Chrome has always been somewhat of a leader in modern UIs, and their carefully crafted preferences interface is testimony to that fact. Notice how, when you search for "ser", Chrome searches even inside the Content Settings box. This is what user interfaces should aspire to be: clean, functional, and efficient. Hats off to the Chrome team for a job splendidly executed.
Next we have our friendly neighbourhood operating system, Ubuntu.
Ubuntu HUD: this is what you could expect in Ubuntu 12.04.
Ubuntu, come April, will sport an SUI on every single application in the OS. They call it the HUD. The HUD will allow you to search through menu options in a jiffy, so you can say bye-bye to the GIMP's messy menus, and instead search for what you want to do. Of course, the HUD cannot substitute menus, because SUIs have terrible discoverability (i.e., you cannot browse through available actions, you must know what you're looking for); the best solution would be to have both, traditional menus and the HUD.
Do you know of any other SUIs? What is your opinion of them? Let me know in the comments.