Anime Ratings Legend
1.0 - Terrible, Atrocious, Horrific1.5 - Bad
2.0 - Below Average
2.5 - Average
3.0 - Above Average
3.5 - Good
4.0 - Very good
4.5 - Superb, exceptional
5.0 - Masterpiece and/or personal favourite
Anything rated 4 or above is well worth watching. Anything rated less than 3 is probably not worth it unless you're big anime fan with some spare time, in which case you'll probably want to watch 2.5 (and in some cases even 2.0) as well.
Keep in mind that these ratings are personal. Very few shows will get rating of 5, several shows will get 4.5 and quite a few will get 4. Majority of stuff will be rated less than this. I tried to spread ratings evenly, taking into considerations that most shows I've seen are highly rated by review sites and fans - that's how I decided what to watch in the first place. In other words, most anime I've seen is not bad because I already filtered out most (but not all!) of truly bad stuff so there is going to be a skew towards above average/good.