If I were to hire a WordPress developer, my wish list would look like this:
- Knows HTML and CSS inside and out;
- Knows their way around PHP, and produces beautiful code (clean, consistent, DRY)
- Has a good grasp of database design and SQL: if I show you a database schema and a few queries, can you tell whether they’re well-designed and efficient?
- Understands their .htaccess file;
- Understands The Loop thoroughly;
- Understands how taxonomies, custom fields, custom post types, and post formats work, and what they’re for;
- Knows how themes work and understands the template hierarchy;
- Knows how plugins work and how to write a simple one.
There’s a lot more I can think of, but I think the above would be a pretty solid start.
And here’s a trick question: what sucks about WordPress, and what would you change in the next major version to fix it? If you can give a convincing answer to this, you probably know quite a bit about WordPress.
But setting aside the above wish list for a second, the most important thing to me is not whether you know the solution to a problem: it’s whether you can find the solution.
Know where to look (the Codex, the PHP manual, places like this), and be genuinely interested in doing things the right way. Don’t be satisfied when the thing that didn’t work suddenly works: was that the right solution? Could you do it in a more elegant way, with less code? Can this solution scale, can it be reused when a similar problem crops up?
Hope this helps.