Buying too many of the same variety. I used to be a Cabernet Sauvignon lover, but bored of it over the years. I wish I would have branched out from the get go.

Well, I started a cellar at 20, so it wasn't a matter of being "late". Apart from that I have committed and regretted all of the same things as above. Now it falls into two "regrets"

The first, I chose viticulture, though warned by my professor, I stuck to this thread which paid about half the salary of a "winemaker".

The second, having less disposable income means being unable to buy an eclectic range of wines. I am owrse now at tasting European classics than 30 years ago ... they simply cost more now.

On second thoughts, I don't "regret" my trade, I wouldn't change. Poor but content.

Ratings have a place. That being said, I've had plenty of low rated wines that outperformed expensive highly-rated wines. With older vintages this is especially the case, as some of the best wines I've had have been rated in the 70s and low 80s (even 64 in one memorable case). At this point I'm convinced that few critics (Broadbent being a notable exception) can really predict how a wine will mature. While two years ago I never would have bought a lower-rated expensive wine, I'll occasionally do it now if I have confidence in the provenance and a producer.

My cellar fluctuates too much in temp. I might have to go to storage now that I have alot of great wine in my cellar. I study wine everyday and I used to work in a great wine restaurant, now I work in a casual dining restaurant and run a small wine list. I have a great nose for wine but my tasting lacks a little behind when I taste huge wines. I study different varietals all the time. Ican distinguish cabs to beaujolais, rhone to piedmont. But the more I learn about wine the more I find I don't know about certain regions or grapes from around the world. I'll never give up.

not enough cab or german reislings

1) Not enough whites. I now love aged chardonnay and riesling, but have very small amounts of aged whites in the cellar.
2) Aging cheap wine. All what I ended up with was old cheap wine.

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The ultimate result of shielding men from the effects of folly is to fill the world with fools. -Herbert Spencer

Not being born into a Forbes 400 family so I could afford the wines to which I would like to become accustomed.

Seriously, though, the regrets I have are the bottles I was tempted to buy, but did not.

Building verticals. What a waste of time, particularly for those of us who drink (as opposed to merely collect) our wines.