Hi, Naomi. I've been using Wordpress off and on for years. My current site runs on a Wordpress engine with a (modified) free template, but I have constructed full templates from the ground up.
The biggest challenge, to me, with a Wordpress template modification is ciphering out just how this (data) got there (markup), then modifying (or writing new) css so that your site fulfills your vision or projects your cultivated image. Once you've figured out how the template's author put it all together, thought, it's fairly cut-and-dry. Fitting newly-created custom images into an existing template seamlessly can also be a challenge if you are trying to make dramatic changes.
Keeping in mind the fact that you don't have to re-invent the wheel goes a long way toward making Wordpress a viable option for site-operators/owners who don't have a lot of experience in web design (or any real desire to acquire that experience) but still want a professional, high-quality site without the expense of hiring a professional web designer.
Personally, I think that blog engines (especially Wordpress) are excellent tools for anyone who has little or no design or development experience and wants to publish, manage & maintain their own site. One of the reasons I feel this way is the fact that features can easily be added and removed even after the site is launched without causing a major reworking of the code... Just turn features on and off as you like. ExperiencedGirlfriend.com is certainly a living site that is able to adjust to whatever is working for me on a day-to-day basis. That's comfortable to me even though I don't quite have it all just like I want it yet. I know it will be my ideal before long; it will just take time.
Naomi, I'm sure once you've gotten into the administration panel and played around a bit you will quickly come to appreciate how intuitive and accommodating this engine can be. If you fit any snags or just have a question, from huge to tiny, please feel free to ask me and I will do my very best to help you in any way I can.
Best wishes,
Caitie Mae
~sweetness~