I was originally posting a response to this related question, but I found myself on a complete tangent that I feel is appropriate for discussion.
For the purposes of this discussion, I'm using "dependent tag" to refer to any tag which should be dependent on another tag to provide context for why the tag is on the question. Thus, a "dependent-adjective tag" is a dependent tag that is an adjective. For example, multidimensional is a dependent tag. Another tag is useful, otherwise people need to do extra digging to find out whether your question is about XD arrays, hashes, databases, or space. There are certainly others but I haven't researched them.
Not all tags that happen to be adjectives are dependent-adjective tags. Consider canonical. There's actually a "canonical" property in http, so this tag actually stands on its own. The ones I'm concerned with are the words that do not have a special meaning in a programming context.
I think dependent-adjective tags are detrimental to SO's tagging system.
1. They are insubstantial in programming
If I saw a question tagged multidimensional with no qualifying tag, I would have to stop, think "Multidimensional what?", and subsequently look harder at the question. If the title didn't directly state it and it wasn't covered in the first two lines, then I would have to open the question. Extra work is bad, it makes some people who might have been able to help avoid questions, and it aggravates people who wasted their time investigating a question they cannot do anything for. Concepts like strings and inheritance have a very specific meaning in programming. multidimensional has no diversion from the meaning outside programming: something that extends past a single dimension.
2. They do not possess independence for general topics
Unlike nouns and verbs, an adjective needs a noun to describe. You can't just ask a general question about multidimensional. See, I have a hard time leaving that sentence in that state because it is incomplete and it is driving me nuts. Without the noun, it's very likely to be off-topic.
3. They dilute searching
Dependent-adjective tags dilute our searches in two fold. First off, because we do have adjective-noun tags like multidimensional-array, we end up with a bunch of questions that should be found under multidimensional-array, but aren't because they're just tagged multidimensional. Second, it adds an extra tag, and we only have 5 tags to begin with. If you've used up 4 tags already, having the dependent-adjective tag gives you one more confounding option if you haven't used its dependency. In short, dependent-adjective tags reduce the coverage of searches.
3a. We have complex searching algorithms with wildcard searches
A subpoint to 3., in case someone asks "But what if I want to search for general multidimensional content?" or similar questions. If you want XD content, then just do a wildcard search with a multidimensional prefix. We aren't restricted like some places where you have to match the tag exactly. Wildcard searches let us use categorized tags without restricting against looking up general information.
4. Their ambiguity potential is greater than other tags
Consider personal and common. The former is used on SO to describe two entirely different things: questions that are personal (usually paired with subjective), and questions regarding personal content (like profiles on websites). The latter refers to things which are common in occurrence (frequency) and things that are shared in common as well.
I'm certain I've more beefs with them, but these are some of the primary reasons. What does everyone else think?
Update - Additional Finds
I searched from page 100 upwards to page 50 of the tags list on SO to produce the following. These are tags which I undeniably consider to be dependent-adjective tags. I skipped a few tags which I did not have enough experience to judge their dependency. I also didn't note a couple tags which were outside the scope of this request and I would be best off discussing in a separate question.
- optional (18) - Though smaller in number, I find that this is actually a worse tag than multidimensional is.
- encrypted (15) - Could probably be merged into encryption.
- technical (18) - Hejudas, everything on the site is technical information! Things like technical documentation really should be tagged technical-documentation.
- common (19) - Way too ambiguous.
- failed (19) - While I would normally ignore past-tense verbs, that something "failed" is very general. Way too dependent on the question context to provide any meaningful search information.
- regional (21) - Regional nature is either implicite to another tag (cultureinfo, localization) or should be a composite tag like regional-settings.
- full (21) - Between full names, full text searches, and full motion picture... yeah...
- digital (21) - Same deal as technical. There are a lot which should be digital-certificates.
- conceptual (23) - Ambiguous between the question and content.
- special (27) - If I knew this tag existed, I'd make sure all of my questions were understood to be special. Joking aside, the majority of these fit with the existing tag special-characters
- strange (30) - And with death aeons even strange may die.
- persistent (38) - The majority should be merged with the tag persistence, but there are a couple cases that aren't about the programming concept of persistence.
- absolute (40) - Ambiguous between absolute values and absolute paths at the very least.