There are a number of features which are difficult to use without the UI from the Mac version, and some which are impossible to use except on the Mac. Some of the features where the Mac UI comes in handy can be accessed with a little bit of clicking on magic URLs and so on.
Features enabled by Mac UI: (many obeyed by iPad/iPhone, but not visible)
- building your own perspectives (custom displays of tasks)
- simultaneous manipulation of multiple items
- control of Inbox behavior (actions need project/context/both before auto filing)
- selection of default single action list
- selection of default due time
- selection of default parallel/sequential project preference
- selection of default auto-complete/no auto-complete project/action group preference
- selection of per project/action group behavior on completion
- editing/displaying duration estimates for tasks (can't edit, or display directly on iOS, but can be used to filter perspectives)
- editing next review date for a project
- editing completion date for an action or project
- viewing added or changed date for an action or project
Features only provided by Mac version:
- archiving of completed tasks/projects
- Applescript support (reports, extended searching, etc.)
- Clip-o-tron conversion of email into actions with working links to original message
- intelligent clipping support other than sending page URL
- perspectives based on Project view, not Context
- automatic database backups
- automatic creation of actions from incoming email
- display of client sync status
- syncing of actions to iCal todo lists
- drag and drop attachment access
No doubt I've omitted some things from both lists, but it should at least give you an idea of what you are missing.