Excerpts and wikis like the first one you mention are completely redundant :). The purpose of a tag is to indicate that a question involves subject matter in that topic - questions with the resistor are by definition "Questions on/about/relating to/using resistor." The "in circuits and designs" part of the comment should be obvious based on the fact that this is the Electrical Engineering StackExchange site. In my opinion, this is a bad tag in general, but that's another meta post.
Here are some guidelines that (IMHO) a good tag wiki excerpt should follow:
- Brevity. The first sentence (or two if they're short) should give the essence of the tag. In some views, only the first sentence or so is shown, leaving the rest for the more curious. A long tag excerpt is less likely to be completely read - save less significant details for the actual wiki.
- De-Jargon. The op-amp is a good example, since it mentions its root term (operational amplifier). If the tag is an acronym, the excerpt should give the full term. This reduces potential confusion and helps novices in the subject area.
- Well written. Answers should use correct terminology and not mention the purpose of a tag. (e.g. Questions relating to... )
- Stand-Alone. Don't make comparisons in the tag excerpt, there isn't enough room. Describe a topic fully. For example, the analog excerpt says "Analog circuits have a range of voltages, rather than just two as in digital logic." which illustrates a dichotomy instead of describing "analog".
Any in-depth information should be contained in the tag wiki. Ideally these would be two different bodies of work. A good place to see what the tag wikis and excerpts should look like is the StackOverflow tag page.
That having been said, our tag system is in fairly poor condition. We have a lot of tags that are add-ons that don't improve the categorization, and we are doing a poor job of culling new and underused tags, and properly categorizing new questions. Some tags, such as voltage, current, power, resistor, etc. are so broad and all-encompassing that adding them means nothing. When was the last time you saw a functioning circuit that didn't have voltages or currents?