All the reviews I could find for the first 4 of 8 episodes of the season:

http://www.empireonline.com/tv/defenders/
"And, we’re sorry to report, the big threat that unites the group proves a mild disappointment. Alexandra feels like yet another businessperson baddie whose dirty deeds are hidden by sharp suits, while the ongoing, vague chicanery of The Hand (like HYDRA, but ninjas) fails to ignite much excitement. Indeed, the peerless Sigourney Weaver herself seems desperately bored by it all.
Even so, there is enough joy in watching the sparks fly as these four alpha characters finally collide to make you feel The Defenders might just have been worth the wait. It's a long-anticipated crossover series that delivers some engaging character clashes, but which fails — in the first half at least — to up the ante story-wise."
3/5

http://wegotthiscovered.com/tv/defenders-season-1-review/

"All in all, The Defenders is a solid addition to the Netflix branch of the Marvel Cinematic Universe. While not as revelatory as The Avengers (that kind of a game-changer can really only occur once), the show likely presents everything fans have been hoping for. From a narrative and thematic standpoint, The Defenders feels like a turning point for each of these characters, as their four paths intertwine in the face of their biggest challenge yet. From a production standpoint, the show has flaws — bizarre camera angles, inconsistent fight choreography, etc. — but none of that’s a deal-breaker, especially when you’re dealing with characters as complex and fascinating as those at the center of The Defenders."
4/5

http://collider.com/the-defenders-review-netflix/#images

★★★ Good — Proceed with cautious optimism
"If you were hoping that The Defenders, with its short 8-episode run, would rectify some of the issues of the solo series regarding pacing and narrative choices, it does not. But like in those series, there are still things to enjoy."

http://www.thisisinsider.com/the-defenders-netflix-review-2017-7

B (This may be generous, but it's also after only watching half of the mini-series which has the potential to get equally better or worse.)
"When they all finally come together, fans may be sad to learn the mini-series will already be halfway over."

https://www.washingtonpost.com/news/comic-riffs/wp/2017/07/24/the-defenders-is-the-marvel-team-up-you-hoped-for-iron-fist-and-all/?utm_term=.1070c76930a7

"Well worth the wait, the new show is every bit the event that Marvel fans hoped it could be. And so it seems Marvel’s connected Netflix universe is back on track with no apparent “Fist” hangover. The biggest question going into this street-level hero team-up was how four shows with such different vibes would come together as one. The best part of “The Defenders”? Watching how they make that happen."

http://decider.com/2017/07/24/netflix-marvels-the-defenders-review/?utm_campaign=SocialFlow&utm_source=DeciderTwitter&utm_medium=SocialFlow

"Ultimately, The Defenders had to successfully accomplish one huge thing: justify 65 episodes of build-up. Despite a slow start, the show comes through on its promise and gives the heroes a threat worthy of their newly united attention. Defenders also doesn’t sacrifice the vital voices of Jessica Jones and Luke Cage when it throws them in a very Daredevil-y plot, and Defenders even makes Iron Fist work. These characters work so well together, it’s now hard to imagine them apart."

http://io9.gizmodo.com/the-excellent-defenders-is-greater-than-the-sum-of-its-1797180699

"However, the good of The Defenders far outweighs the bad. The story isn’t revolutionary, and as usual, the villains have a typically over-the-top and hilariously convoluted plan—but it’s all a canvas to showcase Matt, Jessica, Luke, and Danny coming together as a team. They’re not just stronger together than they are apart—it turns out they’re more entertaining, too."

https://www.cnet.com/news/the-defenders-review-marvel-netflix/

"'The Defenders' is the superhero power-group payoff we've been waiting for. Personal preference and style influence your favorite Netflix-Marvel show, but this one has something for every fan and makes some of the solo stories pale in comparison.My wishes for the last four episodes: continue the snarky banter; give us more about mysterious Alexandra and the organization's backstory; and keep up the pace. Oh, and save New York."

http://www.ign.com/articles/2017/07/26/marvels-the-defenders-chemistry-rivals-the-avengers-season-1-first-spoiler-free-impressions

"The first half of Defenders is off to a solid start. It does take a tad too long to get going, and it has a lot of work to do in order to make Iron Fist palatable, but it balances the story well as it slowly reveals an intriguing new villain and pushes the four heroes into doing something they very much do not want to do: work together."

http://411mania.com/movies/marvels-the-defenders-review/

9/10
"The Defenders is so far off to a promising start. The return of all these great characters is fantastic, but the heroes getting to meet and interact with each other for the first time is equally satisfying. Sigourney Weaver's Alexandra, despite being steeped in mystery, might turn out to be the show's most interesting character. Consider the rating a bit of a placeholder, and that rating might change after the rest of the season has been released. Once, The Defenders gets moving and Iron Fist meets Luke Cage for the first time is where the show really starts taking off."

https://www.bleedingcool.com/2017/07/24/the-defenders-review-team-chemistry-makes-up-for-a-slow-start/

8.5/10
The Defenders is off to a solid start thus far, and while it’s hard to say whether the story is going to work all the way through, the first half is engaging and leaves you wanting more. While there are times during those two and a half episodes that you’re going to be wishing everyone would just meet, the payoff is pretty good.

http://www.radiotimes.com/news/2017-07-25/marvels-the-defenders-first-look-preview-theres-no-meh-in-team

"It’s not a disaster like Iron Fist, but it’s not the triumph that Jessica Jones or the first series of Daredevil were earlier. Like the team itself, when these series are brought together they bring all of their strengths and weaknesses along too – so while we get the good points like Jessica’s humour and Daredevil’s fight choreography, we’re also stuck with Danny Rand being slightly annoying, Daredevil’s supporting cast acting like wet blankets and the supernatural, arc-y explanations being vaguely embarrassing for everyone involved."

http://www.comingsoon.net/tv/features/873325-marvels-the-defenders-review-episodes-1-4#/slide/1

"For fans, this is definitely worth a watch, though getting to the actual story might be frustrating for some. When the team up finally does happen, it’s pretty satisfying. If you haven’t seen all the other series, fear not. A lot of time is spent explaining. Hopefully we can let that go in the second half.

http://www.digit.in/entertainment/marvels-the-defenders-review-in-progress-36190.html

"What we loved most about the first four episodes of Marvel’s Defenders is Elektra Natchios (Elodie Yung) and her enhanced combat skills. Her use of the Okinawan Sai, daggers and Katanas lend the classic comic book touch to her character. Since we can’t discuss specific plot details before the show officially releases, let’s just say that there are only two new characters to look forward to in The Defenders - Alexandra and Elektra Natchios. It’s business as usual with the superhero ensemble, and as with Iron Fist, Netflix and Marvel have stuck to the formula of short bursts of energy as the show progresses. Honest to god! there were moments where we (My binge watching partner and I) dozed off due to the lack of excitement and long periods of explanations offered by characters at different intervals in the show."