Farside847, at 'only' 145,000 keywords, you're actually well above the suggested maximum number of keywords for an account.
If you'll permit me to get up an a soap box for a moment, the best answer to your question is really provided in the message that you've quoted:
If your account is along the lines of many that I've seen lately, you'll probably have at least several thousand keywords that are getting no impressions, or no clicks, or which are appearing on the 24th page, or which are not really related to the product or service that the Ad Group is offering, etc. Looked at another way, if just 5% of your keywords were so-so, that is over 7000 keywords that aren't providing good results.
Getting rid of them will open up a lot of spots for keyword that you want to add, moving forward.
A very good place to start is to delete all keywords that are currently disabled, or which have had just an impression or two over a period of weeks. With many millions of searches per day done on Google, one or two impressions over two weeks is a pretty good indicator that the keyword is not successful.
At the bottom line, the message you've received is just asking you to look at your 145,000 keywords, and delete the ones that are low performance/low value. The reason you see the 'new error' message is that vast lists of low value keywords have the potential to slow the AdWords system down for all advertisers - which I hope you will agree is not a good thing.