I started brewing recently and have made three batches from extract kits (details below). I had a little experience before this in helping my dad make mead and wine, so I at least had a clue when I started. I sanitized and followed all directions religiously, and I ended up with three beers that should taste pretty different from each other but all taste incredibly similar. There are no off-flavors. The aroma is very nice on all of them, and the color, head, etc. all look right. OG and FG were exactly where the instructions specified. But they all have a very thin flavor profile and/or seem overcarbonated, with only subtle notes of the beer's intended flavor.
#1) Brewer's Best Scottish Ale, OG 1.034, boil volume 2.5gal
#2) Midwest Supplies Hex Nut Brown Ale, OG 1.043, boil volume 5gal
#3) Midwest Supplies Irish Stout, OG 1.047, boil volume 3gal
After the first one came out thin but with everything else seemingly on-target, I figured I needed to steep the specialty grains for longer (30 minutes on batches 2 and 3 compared to 20 on batch 1), which resulted in little to no improvement. All three seemed to have a much fuller flavor just before bottling compared to after bottle conditioning - going into the bottles each batch tasted great, and all three notably different from each other as they should be, but after 2 weeks in the bottle they all taste very similar and seem to have lost their flavor. There might be a tiny improvement in the flavor after extra time in the bottles (a month or two), but it's small enough that I could just be fooling myself.
I'm using secondary fermentation (roughly a week in the primary and a week in the secondary), bottle conditioning with the 3/4 cup of priming sugar that is included in the kits, and the fermentation/carbonation temperature is around 65F.