You should use hyphens rather than plus signs if possible.

Plus signs are a "reserved" character normally used in dynamic URLs and could cause confusion for the web crawlers which could lead to pages not getting indexed.

However, if you have been using plus signs and don't have any issues with those pages not being indexed, you can probably safely leave them in place. There are sites that use plus signs without any problem.  If those pages already have lots of inbound links, you would need to do 301 redirects or use a rewrite to change the plusses to hyphens. Apparently plus signs can be recognized as separators like a hyphen, so there is probably no significant difference in regard to SEO.

So if it isn't going to be a huge hassle with redirects etc, I would recommend changing to hyphens.

If most of your posts have the same tags and categories, your tag and category archives will look a lot each other and look like your main page which could lead to duplicate content issues.  Many people choose either tags or categories to be indexed and the other blocked. If you can make your tag or category archive pages excerpt only, they will be good for navigation and not be as likely to look like duplicates.  Like EGOL said, category pages can be a pretty big draw - especially when they are full of excerpts that are keyword-rich.