Well... you got some nice, natural expressions. The posing is not particularly good, but not too bad... no "forced" look.
The first thing to work on is lighting. All of these are lit very flat, with some very hot light spilling into the first one. It takes a lot of study and experience to develop a good outdoor lighting approach.
The one easiest thing you might have done for the most improvement is a change of wardrobe. I'm sure they were happy with their clothes, but these outfits are doing nothing good for their appearance or your photograph. The clothes are demanding all the attention, drawing the eye away from their faces and exaggerating their body size.
1.the overcooked eyes on the woman is REALLY creepy. She looks possessed. Sorry. She's rather underexposed and he's a bit over exposed. I am guessing you had lens flare on him and had to boost the blacks to recover this. It REALLY should have been shot with flash. Both subjects would have been in better exposure and your sky would not have been white. You should definitely tilt, but it is not a good tilt when it makes you want to tilt your head or the image to balance it. This one doesn't work. The posing is really not so hot in a couple of ways. They have no connection to each other which is KEY in an engagement photo. This is your typical Olan Mills engagement or couple photo pose. The flower ring should have been covered or gone to highlight the engagement ring. With larger subjects you ALWAYS want to be above eye level to flatter their weight. You have effectively made both of them larger here. The double chins are a big deal and you've brought them out well. When you shoot from above you hide those beneath the real chin-as well as a lot of other things that don't get seen. This image is rather purple/blue in color on the face.
2.Well, they have some connection on this one. You've done a nice job of really accentuating his belly in this one. The hand chop is really unfortunate, but the image can be salvaged to get the two faces gazing at each other. Exposure is much better on this one because the lens flare didn't happen to make one lighter than the other. Your tilt isn't AS bad on this one, but really shouldn't be there. It's a distraction. So is the thing above her head. Color looks good and exposure on your subjects is good.
3. It's the best out of the lot. Again should have been shot with flash. The angle you shot from really accentuated double chins. His hands gripped like that in front of him is a really negative statement for an engagement. It also REALLY accentuates the belly. The engagement photo isn't about posing for something like Olan Mills. It's about the emotion and connection between two people. The only connection here is between the woman and the camera. Flash, again would have prevented the white out sky.
There is no colorspace embedded in these so I am guessing that we are seeing a different color than you are on your computer. you want to be working in sRGB and making sure it's embedded for internet use or nothing will appear the same.
The vignetting is not horrific in these, but it does give a bit of a cheezy look.