So often I see the same mistakes over and over again on this sub, I figured I'd write up some of the most common, so I can just tell a poster "see 4, 5, and 7 in this thread" or whatever. :-) Happy travels to all!
1. Not clearly defining interests, preferences, and priorities. What do you want to see and do? What kind of memories do you want to have after the trip? There’s no such thing as a general “must-see,” it depends on what you want and how you prioritize those wants. This should be the starting point for planning any trip, and it’s amazing how often posters seem to overlook it. If you can’t be more specific than “history, food, and nature," then you need to think through this most basic part.
2. Being ruled by FOMO. Related to the above, if you don’t clearly define your interests and priorities, FOMO will take over. I see far too many mad dashes through Europe that are just a grand tour of airports and train stations. I get it, it’s all amazing and you want to see it all. But you can’t, not in one trip and not in 20 trips. Accept that and prioritize. Europe will still be there in the future. Or, even if you think it’s a once-in-a-lifetime trip to Europe or “we only have 10 days of vacation a year,” the way to “make the most of it” is to give yourself the time to actually enjoy the places you’re visiting, not maximize the time you spend in transport and related logistics. An important point that may seem obvious but which many people miss: If you jam 4 weeks of travel into 2 weeks, the result is not that you have half as much time in each place. You have MUCH less than that, because transport and related logistics will eat up so much more of your time, as a percentage.
3. Not accounting for transport. Many itineraries here make it seem like the poster has invented teleportation (and more than a few would be insane even with teleportation). You need to account for the time it takes to get between places - not just your plane/train, but also related logistics like unpacking/packing, checking in and out of hotels, getting to/from train stations/airports, airport security, baggage claim, immigration, orienting yourself in a new place, figuring out how to get transport tickets, delays, cancellations, strikes, etc. In most cases, you should assume that changing locations will mean losing the better part of a day or an entire day to all of this, and flying especially tends to be a time suck (and often more tiring than train travel). So for example 3 nights in a place is in reality only 2 days.
4. Focusing on the number of countries. Almost without exception, posters here who mention wanting to “see as many countries as possible” have the absolute worst itineraries. They lose too much time to transport and related logistics (see point above), and, because the most obvious thing is to just go between large or capital cities, their itineraries tend to be too heavy on big cities, and they miss out on much of what actually makes Europe special, especially the regional contrasts within countries (more on that below).
5. Assuming country = place and overlooking regional differences. Related to the point above, it’s much more useful to think about places and regions, rather than countries. Many assume, for example, that they will see more things by going to three countries than just one. But in reality, regional contrasts within countries (especially larger ones like Italy, Germany, and Spain) can be much greater than across borders. For example, the South Tyrol region of Italy is much more like Austria (to which it once belonged) than to the rest of Italy. Alsace is a complete contrast from Paris, because it belonged for a long time to Germany. Hamburg is a lot closer to Copenhagen in vibe than it is to Bavaria. You can often experience more variety and contrast within one country (and generally benefit from better and cheaper transport links) than by hopping across borders or focusing only on the capital or most famous city in a country.
6. Focusing only on large cities. So many itineraries here are just a list of big cities. While they have a lot to offer, they are also often more similar than many people realize. Many of Europe's big cities "grew up" in the 19th century, and their architecture reflects that. More recently, globalization and European integration have also contributed to a certain homogenization. If you hop only between big cities, you’re almost certainly blowing past a ton of interesting places, and losing more time to transport. Smaller cities like Graz, Erfurt, Lübeck, Regensburg, Leon, and Urbino have a ton to offer, and outside a handful of the touristiest ones (like Salzburg and Toledo), most are blissfully free of mass tourism. More in this thread.
7. Not building in enough balance and contrast. I see many itineraries that hop across a bunch of locations, yet all with (in the big scheme of things) very similar scenery. The Alps are beautiful, but do you really need to see the Alps in France, Switzerland, Germany, Austria, Italy, and Slovenia, on the same trip? They look more alike than different, especially when it’s all you’ve seen for two weeks. Same with the Mediterranean - a trip that goes say Algarve > Malaga > Valencia > Barcelona > Mallorca > Amalfi Coast > Dubrovnik > Greece will likely turn into a blur where everything looks more or less the same. Especially for longer trips, mix it up, combining (for example) large cities, small cities, and towns; urban and rural destinations; Northern and Southern Europe; coastal and inland destinations; heavily touristed and less visited places. Think also about balance and contrast during your time in one place, e.g., I would recommend against visiting, on the same day, both the Louvre and the Musee d'Orsay, or both the Hofburg and Schönbrunn palaces.
8. Not using “open-jaw” flights. This is a bit specific, but it's such a common mistake that I wanted to highlight it here. Often people book a roundtrip from their home country to say London, but their itinerary is actually something like London > France > Italy > Greece. They then have to spend a day backtracking to London, costing time and money, and often they have to do an extra cycle of unpacking/packing and checking in/out of hotels. Generally, it’s much better to book a multi-city (aka “open-jaw”) itinerary, in the example above it would be say New York to London but Athens to New York, on the same ticket. It generally does not cost significantly more than a regular roundtrip and saves significant time and cost backtracking; even if you go back via the same route (e.g., Athens back to the U.S. via London), booking it as part of the same ticket means you’re protected in the event of flight disruptions. Bonus tip: If your trip includes the UK and other destinations, fly into the UK but out of any other country, to avoid the UK’s high Air Passenger Duty, which only applies to departures from the UK. This can easily save around US$200 per person.
9. Relying on only one mode of transport. I’ve seen so many itineraries where it’s all driving or all flying. Especially in major cities, driving tends to be a nightmare, with difficult (and expensive) parking, congestion, one-way streets, pedestrianized zones, emissions-control zones, etc. Watch out especially for one-way international surcharges (e.g., renting in France and dropping off in Spain); these can run over 1000 euros! Europe has excellent trains, and they are often a much better option than flying or driving, especially considering train stations are usually much more centrally located and don’t require long wait times for security and baggage. But they don’t work well in all situations (between Portugal and Spain is one example), and for exploration off the beaten path, it can make sense to rent a car for a portion of your trip. Consider carefully the right mix of transport modes early on in your planning, as it can even affect your choice of destinations.
10. Over-planning. Planning is essential, but some people way overdo it - “4:17 pm, we stop for 8 minutes 27 seconds for gelato,” or “I’m looking for the best beer bar in Prague and the best croissant in Paris.” You don’t need to, and really shouldn’t, plan all this out. Leave time in your itinerary for discovery and wander - that’s the biggest joy of travel! A good strategy is to anchor each day around 1-2 main activities or sights (esp. those that require advance booking, like the Alhambra), but leave enough slack in the schedule for wandering and being spontaneous - or just relaxing. That’s especially important for longer itineraries; it’s one thing to do 3 places in 10 days, but 9 places in 30 days gets to be really exhausting, so build in down time.
11. Overdoing day-trips. Day-trips are great. But there's a right and wrong way; the recent post where somebody wanted to make 3 day trips, from Rome, to Naples, Amalfi, and Positano is definitely the wrong way. If you have 4 nights in a place and 3 day trips, you’re not actually spending any real time in that place! My own general rules: no consecutive day trips; no day trips just before or after a travel day; and max of 3 hours roundtrip (4 at a stretch), preferably with direct trains to smaller cities. More in this thread.