1. What do Defense Rating and Heroic Defense rating do?
It's not fully known but the following is believed. Defense Rating is compared to the attacker's Attack rating to determine how much damage a melee swing does. When there is a level difference this calculation is skewed in favor of the higher level. Heroic Defense rating comes into play here. It is believe that heroic defense will help a player's defense rating against higher level monsters. There was also talk a while ago about heroic defense rating possibly increasing the defense rating cap in aoc gold.

2. How much of an effect do the improvements to frenzy stance make
Maxed out Improved Frenzy stance will increase the damage of the frenzy stance by 10%. Guarded Frenzy will reduce the immunity penalty by 10% for each point. Maxed out frenzy stance provides a guardian +60% damage -70% immunities.

3. How much regeneration does Lifesaver, Toughness, Recovery Give at level 80?
Recovery gives around 173 regen at 80 (need confirmation but it should be accurate). Lifesaver and toughness seem to give about 1/3rd of the regen that recovery does for any given level.

4. How much damage does counterstrike and anticipation do at level 80?
A very small amount that scales with level. I don't have any hard numbers but 13/6 respectively at level 45 is what I've heard. So I'd expect these talents to do about 5-20% damage of a normal melee swing for any level(aoc gold).

5. Where should I spend my skill points?
See guardian abilities for descriptions of all skill types

This depends on your focus and server type. Regardless of setup you will at some point need climbing to reach otherwise inaccessible points so plan ahead for that. Other than climbing, if you are on a PvP server, having enough in hiding to hide in broad daylight (about 300-400 points) is a great investment. Endurance and Run speed also provide small, but noticeable, boosts and are a potential area to place points for both faster traveling and PvP effectiveness. Keep in mind that the Run speed does not work in combat however. The resting skills (bandaging, recovery, fast recovery, etc.) are also very important. In addition to providing much less downtime, these skills can be useful for PvP when you can duck out of combat and recovery a few big ticks of health and stamina before jumping back in.

Then there's threat. This one to me is an all or nothing. If you are going heavy PvE then max it out. The more threat you generate the better. If you aren't then leave it at 0. If you're doing a 'hybrid' PvP/PvE build or spec I'd still recommend getting this as high as you can. Take this over endurance, run speed or, if you really have to, hiding. It can be tough to hold aggro in PvE so every little bit really helps.

6. Does plexus strike restore a flat amount of stamina or does it depend on the number of things I hit with it?
Plexus strike will give you additional stamina per target hit by the combo.