Repetitive dates and mapping them onto a calendar is a very complex thing.
To cover the possibilities you need to have settings for every week, every n weeks, every month, every n months, then within weeks allow for the days of the week to be set and in the month allow for the date of the month and the day position of the month (i.e. 2nd Saturday). Then on top of this you need every year, every day and the possibility to set different times within the days so you can build up realistic, usable repetitions such as '7pm on the 1st Wednesday of every month' or 'weekdays at 12pm and 3pm'.
One of the big problems is that it's hard to reduce this complexity without reducing the functionality so really the interface needs to be powerful enough to allow for all these possibilities. You won't be able to make it more understandable by tweaking the options if you want to retain full power and flexibility, however ...
I would do the following four things:
A problem is that it's difficult for a user to visualise the pattern they are creating when using this sort of interface. Have you considered giving the user immediate and clear feedback as they make their selections? For example, have a little calendar on the form that shows them what days they are going to hit with their proposed repetition. [see aside]
Reduce the initial options. Don't show the month related options until the user picks to repeat monthly, likewise the week related options. Hide the complexity until required in order to avoid the form being overwhelming
Get user feedback and analyse real use - so you can put your most popular options first and set good defaults for your current and future user base. And, let advanced user manage their own defaults themselves.
Reduce the need to even open the form use your user feedback to provide common repetition patterns they can use and let them copy the repetition patterns from things that already exist
[aside] As an aside, you might find this very handy while developing, anecdotally from experience date repetition is very tricky