Even with registering, you may well end up with a pile of stuff off-registry. I don't know why our guests thought that we needed a truck full of vases and picture frames, especially since we didn't register for a single one :p Some we kept, most we donated.
Good things to register for:
-You can never have too many towels (remember beach towels and dish towels too), sheets with extra pillow cases, and pot holders. If you have the space to store them, they come in handy when your current ones wear out.
-Blankets and throws. On the bed (obviously), on the couch, in the car to protect the seats/carpet from messy items (think bringing plants home from the nursery), in the closet for when guests come over.
-Nice cookware. Upgrade what you have to nicer, tougher items. I'm thinking cast iron pans, heavy ceramic cookware (Le Creuset and the like), anything that looks like it will last you the next 50+ years rather than the next 10.
-Disposable cookware. Rubber scrapers, wooden spoons, plastic spatulas, pastry brushes, vegetable scrubbers, sheet pans, cooling racks, tongs - those little things that may last you 5-10 years or so, but will wear out eventually. Either upgrade your almost worn out items, or put some aside for when your current ones finally kick the bucket. I've seen lots of gift baskets given filled with these little items, a couple dishtowels and maybe a cookbook.
-Sports & camping gear. We registered for, and got, kayaking gear and sleeping bags - some of our favorite gifts! We also received a National Parks Pass which we were able to register for through REI. Replace camping gear that is wearing out or upgrade to something better designed than what you are currently using. Tents, sleeping bags, camping chairs - they are all so much nicer now than they were 10 years ago. Definitely worth looking at.
-Tools. Nothing wrong with borrowing from friends and neighbors, but if you don't have the basics it is really nice sometimes to have your own.
Things I (or friends) regretted registering for:
-Looks like fun, but you've never owned one before, appliances: Fondue set, ice cream maker, juicer, fancy mixers/processors, Panini press, deep fat fryer, etc. Unless you KNOW you will use it a million times, don't bother. It just clutters your house and honestly makes you feel a little bad that you asked for it. I feel guilt over our deep fryer. We used it twice in over 10 years :p My friend got a Cuisinart mixer that she had no room for so it sits in the garage. It is so heavy to move, its too much bother to go get, so she just uses her little hand mixer that she had pre-wedding.
-China/Crystal/Silver: It only gets used a few times a year, its a huge pain because it can't go in the dishwasher, shouldn't be stacked, shouldn't have acid foods left on it for any length of time, etc.. Oh - and of course they discontinued my patterns so if anything gets broken I can't replace it. Oh - and if you really get off your rocker (like I did) you feel like you need the complete set and buy DUMB pieces. My family is not, never has been, the "coffee after dinner" crowd. Guess who has a creamer and sugar set that has been rotting in boxes for over 10 years?!? Yep. Dumb.
-If you do a honeymoon registry, be very careful with it. A friend of mine got a few excursions partially paid for, but not entirely. If she hadn't already been prepared to pay for those excursions regardless, she couldn't get that money back. With mine, a person very kindly paid to have a bottle of champagne waiting in our room on arrival. With the type of room we had, however, we had a free in-room bar already there. The resort didn't change the registry depending on the type of room booked.